Sunday, November 10, 2013

The Case Against a General-Purpose Cartridge

Despite my efforts to keep to one system, this article hops between metric and imperial units quite a bit. Often, alternate units will be contained within parenthesis following a figure, but it's always a good idea to keep conversion factors handy when working with small arms. I use 25.4 millimeters to the inch, 3.28 feet per meter, 145 PSI per MPA, and 15.43 grains per gram.

Anthony Williams - an internet military enthusiast, collector, and author - is the primary online proponent of the universal rifle/machine gun caliber. The General-Purpose Cartridge, as he calls it, would theoretically combine the lethality and range of a 7.62mm weapon with weapons and ammunition closer to 5.56mm systems in weight. While he never lays out the requirements for such a round in an organized fashion, a clear picture can be formed from select lines in his article:

This indicates that the muzzle energy, weight and calculated recoil of the GPC should be approximately midway between the 5.56 and 7.62 - similar to the 6.8 Remington and 6.5 Grendel.
The bullet's performance at 1,000 metres should be comparable with the 7.62 M80 ball, as measured by hit probability (a function of trajectory, flight time and susceptibility to wind drift) and damage potential (bullet energy and penetration).

Is the GPC as envisioned by Mr. Williams and his fellows possible? Is it desirable? Is it cost-effective? Previously, I have been very taken the idea of a unified rifle and machine gun cartridge for the military. As a result, I am highly familiar with the multitude of variations on this same concept: ranging from .276 Pedersen copycats, to kurz cartridge revivals, to miniaturized big game hunting magnums, to Mr. Williams' ambitious 6.5/8/800. While the focus of this article is primarily to address the GPC, I will argue that, relative to the 5.56mm and 7.62mm cartridges now in service, all of these proposals fail to satisfy at least one of the three criteria. Some are technically feasible and perform better than either 7.62 or 5.56, but not enough better to warrant the expense and logistical disruption needed to field them. Some are technically feasible, but offer no advantage over the existing calibers while being saddled with significant downsides. Finally, some, such as Mr. Williams' 6.5/8/800, are simply technically unfeasible as currently imagined.


The first group of cartridges all greatly resemble (though none improve upon) the .276 Pedersen. These include the 7x46mm UIAC, the .270 Sidewinder, the .280 British, and others. The second-most mature group, many have examples have actually been loaded and fired, and the .280 British was even officially adopted, briefly. Because their performance has been verified, it cannot be said that these cartridges are unfeasible, but is it worth the cost and effort to field one, in light of the widespread adoption of 7.62mm weapons? In a word, no. Even the literature for 7x46mm UIAC shows it's not greatly more efficient than 7.62 NATO, and it doesn't provide any additional capability, so why would military procurement spend millions re-arming with entirely new weapons and ammunition when they have almost-as-good-and-already-in-the-inventory 7.62 NATO machine guns and rifles? Since these cartridges are also almost as heavy as 7.62, they offer little practical benefit and will be largely passed over.


Due to a growing sense of doubt about the effectiveness of 5.56, numerous cartridges that I would characterize as belonging to the second category have been proposed. These include the 6.8 SPC, the .300 Blackout, the 7.62x40 Wilson Tactical, and more obscure cartridges, like the 6mm-223, 6.5x42 MPC, 6.8mm ARC, and 6x41mm SCC. These cartridges all share greater projectile mass and lower velocity than 5.56mm, and relatively small cases, so that existing 5.56mm rifles can be rechambered for them, if necessary.

My analysis of this category will be short: They are a technological step backwards. Their low velocity (typically not above 800 m/s, with some even below 700 m/s) and non-exceptional projectiles (typically with low sectional densities comparable to 5.56mm) produce ballistics inferior to that of the M4 Carbine in terms of trajectory, while offering little to no more energy at range for every pound carried. Most of the literature on these cartridges stresses their superior energy at range per shot to 5.56mm. Usually, this is true, but these cartridges come saddled with so much extra weight compared to 5.56mm that the advantage, if there was any there in the first place, is moot. A short example is as follows:

Trajectory
Input Data
Ballistic Coefficient:0.151 G7Caliber:0.224 in
Bullet Weight:62.0 gr
Muzzle Velocity:2950.0 ft/sDistance to Chronograph:10.0 ft
Sight Height:1.50 inSight Offset:0.00 in
Zero Height:0.00 inZero Offset:0.00 in
Windage:0.000 MOAElevation:0.000 MOA
Line Of Sight Angle:0.0 degCant Angle:0.0 deg
Wind Speed:10.0 mphWind Angle:90.0 deg
Target Speed:10.0 mphTarget Angle:90.0 deg
Target Height:12.0 in
Temperature:59.0 °FPressure:29.92 in Hg
Humidity:0 %Altitude:0.0 ft
Vital Zone Radius:5.0 in
Std. Atmosphere at Altitude:NoPressure is Corrected:Yes
Zero at Max. Point Blank Range:NoTarget Relative Drops:Yes
Mark Sound Barrier Crossing:NoInclude Extra Rows:No
Column 1 Units:1.00 inColumn 2 Units:1.00 MOA
Round Output to Whole Numbers:No
Output Data
Elevation:6.951 MOAWindage:0.000 MOA
Atmospheric Density:0.07647 lb/ft³Speed of Sound:1116.4 ft/s
Maximum PBR:305 mMaximum PBR Zero:261 m
Range of Maximum Height:147 mEnergy at Maximum PBR:740.6 J
Sectional Density:0.177 lb/in²
Calculated Table
RangeDropDropWindageWindageVelocityMachEnergyTimeLeadLead
(m)(in)(MOA)(in)(MOA)(ft/s)(none)(J)(s)(in)(MOA)
0-1.5***0.0***2960.72.6521635.90.0000.0***
1003.93.41.21.12619.92.3471281.00.11820.718.1
2003.21.45.22.32301.22.061988.20.25144.319.3
300-5.4-1.612.63.72006.21.797751.10.40471.120.7
400-24.5-5.324.15.31732.31.552560.00.580102.122.3
500-57.5-10.040.77.11476.11.322406.60.785138.224.1
10/11/13 18:34, JBM/jbmtraj-5.1.cgi


Trajectory
Input Data
Ballistic Coefficient:0.180 G7Caliber:0.277 in
Bullet Weight:110.0 gr
Muzzle Velocity:2550.0 ft/sDistance to Chronograph:10.0 ft
Sight Height:1.50 inSight Offset:0.00 in
Zero Height:0.00 inZero Offset:0.00 in
Windage:0.000 MOAElevation:0.000 MOA
Line Of Sight Angle:0.0 degCant Angle:0.0 deg
Wind Speed:10.0 mphWind Angle:90.0 deg
Target Speed:10.0 mphTarget Angle:90.0 deg
Target Height:12.0 in
Temperature:59.0 °FPressure:29.92 in Hg
Humidity:0 %Altitude:0.0 ft
Vital Zone Radius:5.0 in
Std. Atmosphere at Altitude:NoPressure is Corrected:Yes
Zero at Max. Point Blank Range:NoTarget Relative Drops:Yes
Mark Sound Barrier Crossing:NoInclude Extra Rows:No
Column 1 Units:1.00 inColumn 2 Units:1.00 MOA
Round Output to Whole Numbers:No
Output Data
Elevation:8.926 MOAWindage:0.000 MOA
Atmospheric Density:0.07647 lb/ft³Speed of Sound:1116.4 ft/s
Maximum PBR:275 mMaximum PBR Zero:234 m
Range of Maximum Height:130 mEnergy at Maximum PBR:1155.8 J
Sectional Density:0.205 lb/in²
Calculated Table
RangeDropDropWindageWindageVelocityMachEnergyTimeLeadLead
(m)(in)(MOA)(in)(MOA)(ft/s)(none)(J)(s)(in)(MOA)
0-1.5***0.0***2558.32.2912167.10.0000.0***
1005.34.61.31.12293.02.0541740.90.13523.820.8
2004.21.85.42.32044.51.8311384.00.28750.522.1
300-7.0-2.012.83.71811.31.6221086.30.45780.523.4
400-30.9-6.724.25.31590.91.425838.00.651114.525.0
500-71.3-12.540.67.11383.01.239633.30.872153.526.8
10/11/13 18:24, JBM/jbmtraj-5.1.cgi

We can see the top chart refers to 5.56mm M855 from a 16" barrel, and the bottom to 6.8 SPC from the same length barrel firing 110 gr Hornady BTHP bullets. Higher velocities have been achieved by 6.8 SPC from this barrel length, but not - to my knowledge - with factory ammunition.

6.8 SPC does offer 56% more energy at half a kilometer than 5.56mm (not three or four times as much energy, as I've heard it claimed more than once), but what is that per kilogram? M855 weighs about 12 grams and provides about 34 kilojoules of energy at 500m per kilogram of ammunition carried. 6.8 SPC weighs about 17 grams with a 110 grain bullet, and provides about 37 kilojoules of energy at 500m per kilogram, ten percent more than M855. Is a 25% worse trajectory, increased bolt stress, lower reliability, fewer rounds per magazine, and the introduction of an entirely new cartridge worth a 10% increase in energy per kilogram at half a kilometer? Keep in mind that, due to its large case, good sectional density, and fairly high velocity, the 6.8 SPC is one of the best performers in this category of ammunition.


The third category contains cartridges that either have never been made or are not what they are advertised to be. Examples include the 6mm Optimum, Mr. Williams' 6.5/8/800, and the 6.5 Grendel. Only one of these actually exists - the 6.5 Grendel. To many, the existence of the Grendel proves the GPC concept's viability, but a closer examination shows that is not the case. There are numerous problems with the 6.5 Grendel, first and foremost being its unsuitability for military applications. Beyond (valid) concerns about the cartridge's extreme shoulder angle and lack of case taper, the case itself does not have enough internal volume to accept tracer or steel-cored armor piercing projectiles, except with the lightest bullets. Thus, it cannot fulfill the role of a GPC, as it is not a suitable military cartridge. Further, the 6.5 Grendel provides low levels of performance with factory loads, only achieving its much-touted velocity and trajectory with delicately loaded handloads using very slow-burning and often compressed powders. For field purposes, it is a 7.62x39 with better bullet selection.

Enter the 6.5/8/800, the hypothetical "ideal" general-purpose cartridge proposed by Mr. Williams in his opus. Is this cartridge feasible, and if so, does it offer enough of an advantage to ISAF/NATO members to warrant its adoption and the retirement of both the 7.62 and 5.56 calibers? Mr. Williams' has only provided us with with visual mockups of his cartridges via his website, so the only way to find out is to design the cartridge for him. As per his requirements, the cartridge will be 6.5mm caliber, have an 8 gram bullet, and produce a muzzle velocity of 800 meters per second from a 20" barrel.

Using a combination of CAD modelling and this online Powley Computer, I created a virtual cartridge that met these specifications, so that we can see if it might be a suitable replacement for 5.56mm and 7.62mm.

The first hurdle to jump was the design of the bullet. Mr. Williams specifies that the bullet should be based on the 7N6 projectile of the Russian 5.45x39 cartridge, which would meet both the good form-factor and lead free requirements. However, simply scaling that projectile up yields a bullet weight of only 5.9 grams (less if it is constructed without any lead at all, as the 7N6 incorporates a lead sleeve and plug), far short of the 8 gram requirement. In the end, I designed my own lead-cored bullet, based on the 7N6, to meet the weight requirement, which came to an overall length of 33.5 millimeters, or 1.32 inches for the imperial-unit Powley Computer. One could also use the existing 123 grain Lapua Scenar and get much the same results, since that projectile is 1.295 inches (32.9mm) long. One final note on projectiles: Because of its heavy, high form factor bullets, the GPC already devotes much of its internal and external volume to accommodating the projectile, which, even lead-cored, is already 13% longer than M80 ball's projectile. The lead-free armor-piercing and tracer projectiles necessitated by the cartridge's military application will only aggravate this problem, and may result in the cartridge's weight spiraling significantly to maintain performance. While Mr. Williams does acknowledge this problem, he does not make any attempt to address it.
Fig. 1: The case of the 6.5x50mm GPC, with dimensions

Next, I had to design the case (Fig 1). This was to be a challenge, as utilizing the .30 Remington case (also used by the 6.8 SPC, and favored by Mr. Williams) proved unfeasible due to excessive length needed to attain the requisite performance while maintaining adequate case taper and shoulder angle (.8 and 20 degrees, respectively). I turned instead to the 7.62x45mm Czech for the case head, and attained a case length of 50mm before the performance goals were met. The cartridge now fit within the requisite 2.8" (71.1mm) overall length of 7.62 NATO, and with 45 grains water (2.92 mL) case capacity, was capable of propelling its projectile to 800 m/s within 57,000 PSI (390 MPa) peak pressure, from a 20" barrel (Fig 2).

Fig. 2: The data entered into the Powley Computer

Fig. 3: The completed GPC



With the case and projectile finished, and the specified performance achieved, all that was left was to weigh the cartridge and calculate its recoil. The volume of the brass case was 1.109 cm^3, resulting in a weight of 9.43 g. The weight of the powder charge was 2.35 g (36.3 grains), and the bullet, of course, was 8 grams. To calculate the weight of a large rifle primer, I set five together on my powder scale, weighed them, and averaged the result, which was approximately .35 g. When summed, the 6.5x50/8/00 weighed 20.13 grams; more than two grams heavier than Mr. Williams' initial estimate, and nearly 70% heavier than 5.56mm.

To fully evaluate the cartridge, it was necessary to calculate its recoil energy. The correct formula for this is as follows:


mB = mass of bullet in kilograms  
mP = mass of powder in kilograms 
vB = velocity of bullet at muzzle in m/s 
vP = velocity of powder gas at muzzle m/s  
mF = mass of the firearm in kilograms  
mC = impulse of the cartridge, in kilogram-meters per second 
eF = energy of the firearm, in joules 

The impulse of a cartridge can be expressed as:
mC = mB * vB + mP * vP 

and its energy when fired from a firearm of a certain weight as:
eF = [(-mC)^2]/2mF

The velocity of the propellant gases of modern small arms rifle cartridges is described as being about 4,000 ft/s (1,220 m/s) in this paper from the end of WWII. Using this value, 4kg for the weight of the rifle, the values calculated for the 6.5x50/8/800 above, and 1.69 and 2.92 grams for the charge weights of the M855 and M80 ball, respectively, we can calculate the recoil energy of the GPC, 5.56, and 7.62, for comparison:

5.56x45 M855: 5.9 kg-m/s impulse, 4.3 J energy
7.62x51 M80: 11.5 kg-m/s impulse, 16.7 J energy
6.5x50/8/800: 9.3 kg-m/s impulse, 10.7 J energy

With two and a half times the recoil energy of 5.56, and over one and a half times as much impulse, it's doubtful that it could be an effective replacement for that cartridge in all but the heaviest small arms. While this level of recoil is not too far outside Mr. Williams' initial estimates, I contend that they do not allow the cartridge to be chambered in the light, small, carbines that are now popular, especially in echelon roles. Therefore, the GPC cannot effectively replace 5.56mm.

While this all may sound very negative, there is a silver lining: the cartridge produced about a 20% superior trajectory to 7.62 at 1,000m, along with providing about 15% additional energy at that range with significantly reduced recoil energy. Indeed, while overweight, and having excessive recoil, the cartridge seems superior to 7.62mm in most respects.


Since we have designed a cartridge that meets our performance requirements, and we have some idea of how much it weighs, how would replacing 5.56 and 7.62 with it affect the burden of the infantry platoon? Using the figures from this report on infantry combat loads, we can get some idea.

The infantry platoon in the US Army contains three infantry squads, a weapons squad, and a platoon headquarters.

In each of the three infantry squads, there is the Squad Leader armed with an M4 Carbine, two Team Leaders armed with M4 Carbines, two Automatic Riflemen armed with M249 SAWs, two Grenadiers armed with M4 Carbines and attached M203 GLs, and two Riflemen armed with M4 Carbines.

Weight of ammunition (only, not including magazines or links) of infantry squad breakdown by squad role: 
Squad Leader: 210 rounds 5.56mm, totaling 2.54 kg
Team Leader: 210 rounds 5.56mm in magazines, 200 rounds linked 5.56mm for M249 totalling 4.96 kg (x2)
Automatic Rifleman: 800 rounds linked 5.56mm, totaling 9.68 kg (x2)
Grenadier: 210 rounds 5.56mm in magazines, 200 rounds linked 5.56mm for M249 totaling 4.96 kg (x2)
Rifleman: 210 rounds 5.56mm in magazines, 200 rounds linked 5.56mm for M249 totaling 4.96 kg (x2) 
Weight of ammunition of infantry squad: 51.7 kg (x3) 
Weight of ammunition of infantry squad with 20 gram GPC: 85.4 kg (x3) 
Weight of ammunition of infantry squad with M240s in place of M249s: 85.5 kg (x3)

In the weapons squad, you have the Squad Leader armed with an M4 Carbine, two Machine Gunners armed with M240 GPMGs, two Assistant Gunners armed with M4 Carbines, and two Ammunition Bearers armed with M4 Carbines.

Weight of ammunition (only, not including magazines or links) of weapons squad breakdown by squad role: 
Squad Leader: 210 rounds 5.56mm, totaling 2.54kg
Machine Gunner: 300 rounds linked 7.62mm, totaling 7.26 kg (x2)
Assistant Gunners: 210 rounds 5.56mm in magazines, 400 rounds linked 7.62mm for M240 totaling 12.22 kg (x2)
Ammunition Bearer: 210 rounds 5.56mm in magazines, 300 rounds linked 7.62mm for M240, 140 rounds 7.62mm in magazines totaling 13.2 kg (x2) 
Weight of ammunition of weapons squad: 64.5 kg 
Weight of ammunition of weapons squad with 20 gram GPC: 63.8 kg

In the platoon headquarters, you have the Platoon Leader armed with an M4 Carbine, the Platoon Sergeant armed with an M4 Carbine, the Radio Operator armed with an M4 Carbine, the Combat Medic armed with an M4 Carbine, and the Field Artillery Forward Observer, also armed with an M4 Carbine.

Weight of ammunition (only, not including magazines or links) of platoon headquarters breakdown by squad role 
Platoon Leader: 210 rounds 5.56mm, totaling 2.54 kg
Platoon Sergeant: 210 rounds 5.56mm, totaling 2.54 kg
Radio Operator: 210 rounds 5.56mm, totaling 2.54 kg
Combat Medic: 210 rounds 5.56mm, totaling 2.54 kg 
Weight of ammunition of platoon headquarters: 10.2 kg 
Weight of ammunition of platoon headquarters with 20 gram GPC: 16.8 kg 
  
Total weight of ammunition in the platoon: 229.3 kg   
Total weight of ammunition in the platoon with 20 gram GPC: 336.8 kg 
Total weight of ammunition in the platoon with M240s in place of M249s: 330.9 kg 
Weight increase in the platoon if Carl Gustafs are issued in place of 60mm mortars: 47.5 kg

It is clear that replacing both 5.56 and 7.62 with a cartridge meeting the specifications of Mr. Williams' GPC, as much as that is possible, increases the weight of ammunition of the platoon well above the threshold of other more cost-effective solutions. In addition, 30 out of 38 personnel in the platoon are armed with M4 Carbines. If a GPC were adopted to replace 5.56, those rifles and magazines would have to be supplanted with larger weapons and ancillaries compatible with the new cartridge, adding further weight to the platoon. If being out-ranged is a major concern, issuing more 7.62mm caliber M240 machine guns in place of 5.56mm caliber M249s is a much more cost-effective solution than a complete conversion to a GPC cartridge, while having a similar increase in the weight carried by the platoon. If the problem lies with an inability to use mortars due to restrictive rules of engagement, it would be lighter to issue Carl Gustaf recoilless rifles as direct-fire assets in their stead, than to issue rifles and machine guns chambered for a GPC.

In conclusion, the GPC concept is one that is attractive only on paper. When rigor is applied, the cost of such re-armament in dollars, hours, and pounds is not justified by the new cartridge's performance. Mr. Williams' proposed cartridges utilizing the .30 Remington and 6.5 Grendel cases are unfeasible as they are conceived, and a cartridge resulting from his performance requirements is too heavy and has too much recoil to replace 5.56mm for most applications. The expectation that he could design a cartridge that produced the same performance as the 6.5mm Arisaka but with much lower weight and sized proved unreasonable. These sorts of errors are understandable, as Mr. Williams does not, to my knowledge, have experience loading ammunition.

A final note: Mr. Williams' intentions are good, but his concept seems to rely on an overly optimistic estimation of what infantrymen are capable of in terms of marksmanship at range. He thus perceives infantrymen armed with more powerful, longer-ranged rifles as having greater capability than those armed with 5.56mm carbines, when in fact, for the vast majority, the heavier cartridge and necessarily heavier rifles and magazines merely increase their burden. In short, giving long-range rifles to line infantrymen without additional training is a waste of strategic resources, money, and time. This is not to say that specialized marksmen would not be able to take advantage of such a cartridge, but they would be better served by a specialized rifle firing a cartridge with higher velocity and better hit probability at range, not the inherently compromising GPC.

43 comments:

  1. I have been working on a new proposal for a GPC, the 6.8 ICS. See my posts at:
    http://68ics.wordpress.com/6-8-ics/
    The bullet weights would range from 120 to 150. Recoil would be intermediate between the 7.62 and the 5.56. The gun would not be much lighter than a 7.62 rifle, but more rounds would fit in the same size magazine.

    ReplyDelete
    Replies
    1. Virtually all of my criticisms that are applicable to Tony's GPC are also applicable to your cartridge. Your estimated velocity is much too high for the case volume, and it's not clear that this cartridge would make the rifle squad any more effective, among other things.

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  2. Velocity estimates are based on comparison with the 30 Remington AR, which has about the same case volume and bullet weights. But at this point those estimates are debatable. Bullet weights from 120 to 150, with higher BCs, should be more effective than the 5.56 round. The goal with respect to the 7.62 is only similar effectiveness with less recoil.

    I don't agree with Williams that weight of guns and ammunition should be lowered, as part of any GPC design. Also, I think that an ideal GPC would still not replace the 7.62 for all roles. The increased use of the 7.62, as a response to the limited effectiveness of the 5.56, is addressed by a GPC.

    ReplyDelete
    Replies
    1. Your cartridge cannot meet its design goals at the stated pressure. You are reaching in excess of 2,800 ft/s with 125gr bullets with handloads. Your handloads are very hot; if you measured their peak pressure, I suspect you would find it would be in the 65,000-70,000 PSI range. There was a fellow I corresponded with, who was developing a cartridge based on the .30 Remington case, with a 50mm case length, necked to .277" - very similar to yours, indeed - who could only get 2,650 ft/s with 125gr bullets with safe pressures. I have no doubt you've handloaded better performance than this, I just doubt it was within 55,000 PSI.

      Delete
  3. I have a lot of respect for Mr. Williams but it seems like the goals of the illusive GPC may be out of reach. Yet surely we can do better than 5.56 and 7.62 NATO. Even if it means two new calibers to replace them.

    Also it seems to me that theoretically Mr. William's GPC would still have value as a 7.62 NATO replacement. Although developments like the GD .338 LWMMG seem to suggest there is a call for something heavier than 7.62 NATO.

    ReplyDelete
    Replies
    1. 5.56 and 7.62 will get replaced with something new, someday. As far as the current technology allows, they are close enough, in my opinion, to the optimum to still give many years of service. Certainly something like Mr. Williams' GPC would give better performance than 7.62, but it wouldn't be better enough to warrant switching weapons in that caliber over.

      Delete
  4. A recent NDIA event announced a new program, CLAWS -- Combat Lightweight Automatic Weapon System, designed to replace carbine (12"), assault rifle (16"), SDM/ DMR (20") and SAW / LMG (24") rifles. "Caliber TBD from pending CCS caliber study." So this could be the start of a long process to replace the 5.56 and possibly the 7.62

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    Replies
    1. Well, I hope that study does more to convince me of the need than anything else has up until this point. I remain pretty skeptical that a new caliber is needed.

      Delete
  5. Thank you! Speaking as someone who likes and is happy with their 6.8 SPC guns, it's nice to see someone else agree that it's not the end all be all round for the military. My needs are not the military's needs, as it were.

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  6. With the 5.56 and 7.62, the military preserves the long tradition of having everything--shirts, pants, rifle cartridges--available in two sizes, too large and too small.

    I think they will probably go with the cartridges they have until, eventually, they are ready to switch to caseless. ammunition. Perhaps the question of caliber will be revisited at that time.

    ReplyDelete
    Replies
    1. I doubt caseless ammunition will ever be ready for prime time. It offers only a marginal advantage over advanced cased ammunition concepts (e.g., alloy and polymer), and invites significant technical difficulties (awful barrel wear characteristics, copious propellant waste, powder disintegration, and others.

      Instead, conventional-layout cartridges with new case materials will probably be the next step, and it seems likely to me that those will be modified 5.56 and 7.62 chamberings, rather than a whole new specification. Beyond that, I don't know, but I will note that caliber almost always gets smaller, and velocity almost always goes up, and the only thing likely to buck that trend is having to penetrate future personnel armor.

      More on the limitations of caseless ammo: http://www.dtic.mil/ndia/2012armaments/Wednesday13614JimSchatz.pdf

      More on the 500 year old small caliber, high velocity trend: http://196800revolutionsperminute.blogspot.com/2013/10/small-caliber-high-velocity-isnt-new.html

      Delete
  7. This is actually one of the best articles on this subject I have seen in years. Very few people have an understanding of the soldiers' load, Table of Organization & Equipment, logistics chains, effects of increased performance on weapons weight, and the limitations of marksmanship training.

    I'm a huge 6.5 Grendel proponent for hunting and recreational target shooting, and enjoy the cartridge thoroughly, but I have had to analyze this carefully knowing the realities of dismounted infantry combat.

    Too many people with no infantry experience have been covering down on this topic for decades, with no real practical understanding of what it means to reduce the current soldier's ability to sustain the fight with the load that SCHV cartridges allow. While everyone seems to be fixated on "improving" the individual service carbine, some of the most important weapon systems get neglected, as does the even more consequential subject of training.

    If somebody wants to really offer something that would change the capabilities of the Infantry Squad and Platoon, and can only conceptualize an equipment or weapons solution, focus on the Light Machine Gun, DM Carbine, portable indirect fire assets, and technologies that have been introduced with developments in situational awareness, aiming, and coordinating of fires.

    The more important handicap in the US Army is the Train Fire doctrine and execution of marksmanship training, which is a purposely dumbed down concept from the nuclear era where conscripts were expected to occupy a fighting position overlooking their sector of fire, and hit man-sized targets out to 300m. In an age where asymmetric warfare has ruled the day for the most part, professional marksmanship training has been needed, but systematically neglected by a military bureaucracy that could simply care less.

    5.56 NATO is not our problem, and probably has saved a lot of lives when soldiers would have been left out to dry with larger ammunition that would expend faster in a series of engagements and FRAGO's, continue mission scenarios.

    If we were to replace any cartridge at all, I would lean towards phasing 7.62 NATO into vehicle-mounted platforms, while introducing a 6.5mm LMG and DM into the Infantry Squads, but we have so many years of 7.62 NATO in the logistics cycle, it would be difficult to pull off.

    One of the main family of weapons we need to keep in mind are the PKM and newer variants, combined with SVD and variants, and the challenges they present to a dismounted Infantry Squad + sized element. Thinking in terms of over-matching the PKM and SVD by means of reduced weight, but increased external and terminal performance, requires higher BC projectiles, as increasing the sectional density of the 7.62 NATO leads to more weight.

    A 55,000psi 6.5mm pushing a 130gr projectile in the mid .5 G1 BC region would be the ticket to over-match the PKM and SVD, while reducing soldier's load on existing 7.62 NATO weapon systems.

    The 5.56 NATO should be retained, especially when you look at duty positions and skill sets with the weapons. The DM Program needs to be internalized by each of the combat divisions, since it is now in the MTO&E, and would be fairly easy to accomplish with TNG Detachments at Division Level.

    We will still get it done with the 5.56/7.62 NATO system, especially as 7.62 carbines get lighter, and I would take a sound institutional training solution over a caliber change without thinking twice. The two combined would be quite formidable, but probably beyond the logical capacity of the military institutional trends in weapon and caliber development.

    ReplyDelete
    Replies
    1. Thank you for your well thought-out and detailed comment. I agree that if you were to talk seriously about replacing any small arms caliber in service, one of the first due for retirement would be 7.62mm. Another caliber where perhaps "we can do better" would be .50 BMG, as well.

      Delete
  8. You people are silly sometimes. How much does it cost to shoot these odd calibers? Who has that kind of money. Ammunition costs enough as it is. Yes, reload by all means. It fact it is the only way that one can possible shoot these odd calibers.

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    Replies
    1. For me, costs are the same as .223 Remington and .308 Match, compared to 6.5 Grendel Match from Hornady. This is a military cartridge discussion as I understand it.

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  9. Yes, phasing out 7.62 NATO and .50 BMG with lighter, ballistically superior cartridges would reduce macro logistics burdens in the long run, reduce weapon weight and materials consumption, and make soldiers and vehicles more maneuverable.

    I think the General Dynamics .338 Norma MMG would be a great replacement candidate for the .50 BMG, and something 6.5mm with a 130gr as a replacement for the 7.62 NATO in both LMG, DM, and Semi-Auto Sniper System roles.

    I would also like to humbly submit that the Grendel actually has quite a bit of case taper to it, somewhere between 5.56 NATO and 7.62x39 Russian, so it does feed well even in a design that was not built around it. The 6.5 Grendel is actually full auto tested regularly, and the chamber was meant to accommodate self-loader operation under very dirty conditions when you look at the .300" neck diameter.

    I do believe that a purpose-built powder would drive a 130gr military projectile at 2550fps, using a longer COL than the AR15 is limited to, but the existing design would do extremely well in the DM and SASS roles, with tangible improvements over 7.62 NATO for those shooters. I'm easily getting a very stable 2550fps with the 123gr A-MAX from a 16" Grendel currently with the AR15 bolt limitations, using Hodgdon's CFE223 powder, and didn't see pressure signs on the chronograph until 2650fps, where I had a 40fps departure from the normal 12-25fps increase per .3gr of charge weight, in addition to light cratering.

    I'm very happy with 2500fps with the 123gr anyway, as I have taken that load out to 1200yds at 4400ft elevation, in 80 degree temp, which far exceeded my expectations from a 16" barreled carbine with ~31gr of case capacity. That was with Hornady 123gr A-MAX boxed ammo as well.

    My real concern is how to crack the LMG code with such a short case, without being constrained with 7.62x39 non-disintegrating links for the RPD, which the Grendel already works in.

    The M27 disintegrating link designed for the 5.56 NATO Stoner Model 63 LMG, later picked up by the FN Minimi, can't be used with the current feed pawl arrangement of the MG42/FN feed tray cover articulating system, since the forward feed pawl purchases the remaining forward portion of the 5.56 case wall to pull it into alignment with the feed tray slot. Since the Grendel is quite short, there is no case wall surface ahead of an expanded M27 link to allow that functioning, and using a neck feed pawl in the frontal area would need to be mitigated with the forward feeding motion of the cartridge to clear that pawl.

    That said, an LMG without the COL restrictions of the AR15 magazine well, using an existing case with the capacity of the Grendel, and a 130gr FMJBT or lighter dual-use solid would be an LMG gunner's dream, especially when you consider that he now can have an 11lb belt-fed LMG, with superior wind drift and energy retention compared to the PKM and M240, at less than half the weight of the M240, and much less recoil, longer receiver life, and longer barrel life if the pressures are kept low.

    I would also argue that there is more than enough space for a tracer round, considering the Swedes already had one for the 6.5x55:

    Because of sectional density, the longer 6.5mm projectiles should match or exceed current 7.62 NATO M62 Tracer, and will definitely smoke the M856 tracer in 5.56 NATO.

    The soldier's load capability with a 6.5mm LMG linked system would increase our combat sustainment for Weapon's Squads significantly when weighing the 130gr LMG load versus the M80 ball linked. This is the most logical direction I can see, with the caveat of retaining 7.62 NATO for certain vehicle systems, while phasing in the 6.5 LMG and DM/SASS system to augment 5.56 NATO.

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    1. This comment has been removed by the author.

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    2. Re: Grendel case taper

      According to Municion.org's drawing of the Grendel, it has 7.7 milliradians of case taper, which is more than 7.62 NATO (6.3 mR), and less than 5.56 (8.5mR). 7.62x39 has 22.3 mR taper, which is probably more than is necessary. Here's the drawing:
      http://municion.org/Grendel/65x38Grendel.gif

      However, I will say that accurateshooter's drawing of the Grendel case indicates that it has 10.9 mR of taper, which is actually fairly good for an American cartridge. Municion.org's dimensions are all taken using a careful process of measuring actual cartridges, though, so I'd be inclined to trust its numbers over those of AS. Here's the accurateshooter drawing: http://accurateshooter.net/pix/65Grendel.png

      Further, we can attempt to estimate the case taper of the cartridge by getting a high resolution photograph of it and doing some comparative estimation. We know the bullet is .264" in diameter, and from there I can figure out the approximate case taper. I'll use this image:
      http://www.ammo-reloading.com/images/grendel_ammo-003.jpg

      And from that I get a result of only 4.6 mR! If that result is accurate (and there are reasons it might not be), then the case taper of the 6.5 Grendel might really be something to worry about.

      However, that's far from my primary concern with the cartridge anyway. My concerns mostly center around additional weight and recoil vs. 5.56, potentially reduced lethality with FMJ bullets, a lack of case volume to handle lead-free and tracer projectiles, as well as the cartridge not providing the kind of performance that AA's marketing would have you believe it does.

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    3. I prefer to leave 5.56 NATO as a carbine staple in dismounted infantry units, with sensible projectile and propellant developments. It doesn't need replacing, especially when you look at all the duty positions in an Infantry Rifle Platoon, to include attachments like FO's, Medics, TAC-P's, and so forth. None of those soldiers need anything more than a short-barreled 5.56 M4A2 variant, streamlined to facilitate their primary duties (not shooting).

      My focus is on replacing 7.62 NATO among dismounts with a weapon that is lighter, with better interior, exterior, and terminal performance, using lighter ammunition packaged in user-friendly feeding systems like we had with the SAW Cordura/heavy polymer "nutsacks".

      Regardless of the case taper, the existing 6.5 Grendel feeds and extracts very reliably. Maybe another similar case would work better, but a lot of RDT&E has already been done with the Grendel, and it works very well. There is also successful precedent with 30 degree shoulder cases for military use if you look at the 7.5x55 Schmidt Rubin (Swiss) in the Stgw 57.

      In a belt-fed LMG, we aren't constrained by AR15 magazine well limitations, so case volume increases, and we don't need to seat long projectiles into the case to intrude on it.

      Duty positions that I have held on 10 years of active duty in various Airborne, Infantry, and Reconnaissance units include:

      Rifleman, SAW Gunner, PLT RTO, Grenadier, Ammo Bearer, Assistant Gunner, Scout Observer, Recon Team RTO, Scout PLT RTO, Recon Team Asst Tm Leader, Asst LRS RTO, Scout Sniper, Rifle Team Leader, Rifle Squad Leader, and Weapons Squad Leader.

      I have spent a lot of time behind the SAW, M240, M21, and M24 while deployed in PACOM, SOUTHCOM, and CENTCOM. I would have loved to have something with the 6.5 Grendel's performance in lieu of 7.62 NATO all those years, especially given M118's rainbow trajectory, mediocre wind deflection, and the anemic performance of the SAW on barriers and distance.

      To think that I could have had better performance from a 14.5" to 18" SPR AR15 than the M24, with its .900" OD muzzle and 24" barrel is quite frustrating.

      I can see the Grendel's performance for myself without any marketing, and it does a lot more than one would think looking at the short little case. I've been working with it regularly for the past 5 years, to include use in my Designated Marksmen Courses, and it would make an excellent DM/LMG/SASS cartridge with a 123gr DM/Sniper Load, and a 130gr LMG Multi-purpose load.

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    4. It would be easier, I think, and just as effective from a weight improvement standpoint, to introduce an aluminum-cased version of 7.62 NATO instead of trying to introduce a brass-cased replacement in a different caliber. There's some development needed to perfect this, but it would be much easier on Army logistics when you tried to introduce it into the supply chain (and I don't just mean from an ammo commonality standpoint, but rather from a "we only have one ammo plant and shutting it down to re-tool for new dies, etc is extremely problematic" perspective).

      Here's an example of the kind of weight you'd save by switching to aluminum cases:
      http://www.ncgunowners.com/forum/showthread.php?pid=526836

      Yes, that's polymer cased ammo (which has its own set of problems to work through), but because of its steel case head, it weights about the same as an aluminum cased example. Converted to metric, you get about 18.6g(!) total weight for that PCP cartridge. Aluminum cases, based on my studies, would actually be slightly lighter. For an M80 clone, you could expect weight in the range of 17-17.5g per cartridge, which is lighter than even my most optimistic brass-cased GPC proposals.

      This takes me back, actually, to a 7.62mm replacement concept I drew up a while back, which involved aluminum cases, long steel-cored capped CETME-like 6.8mm bullets, and a disposable chamber throat. It operated at low pressures, which was intended to reduce heating of the MG barrel, so that barrel changes didn't have to happen so often.

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    5. I will say, also, that certainly when some very smart people set about designing a dedicated MG cartridge in the '70s, they came up with the 6mm SAW, which is not too dissimilar to the 6.5 Grendel in terms of velocity and design. So I don't necessarily think the Grendel is unsuitable to that line of work. Is it enough of an improvement? Considering the absolute nightmare you'd have to endure to introduce a new cartridge into supply, I would only say that cartridges of that type have a very high bar to clear.

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  10. Is there a good reason not to use aluminum for rifle cartridge cases?
    Reloading is not an issue for military ammo.
    Is it less suitable as a heat sink? Does it transfer too much of the heat to the chamber before it is ejected?

    I compared aluminum and brass cases in .44RemMag and the aluminum ones were about 60-65% lighter.

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    1. At high pressure aluminum can combust. This doesn't mean that it's a bad choice for rifle cases, but it does need some massaging to work right. The 6mm SAW program of the 1970s experimented with aluminum cases.

      Here's a presentation about efforts today to make aluminum work for military rifle cases:
      http://www.dtic.mil/ndia/2011smallarms/WednesdayM_S12441Mach.pdf

      And here's a very interesting presentation about thin-walled stainless rifle cases:
      http://www.dtic.mil/ndia/2007smallarms/5_8_07/Sadowski_Leng.pdf

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  11. First of all thank you for writing such an insightful article Nathaniel, I really like your perspectives and analysis. As a former operator and armorer this discussion has become a mainstream staple in my interaction with most enthusiasts over the past ten plus years. Growing up I had an interest in aeronautical science, and it led me to the spatial sciences where I now spend the majority of my time professionally. I believe the future of cartridge development lies literally in "rocket science." Unfortunately the development of experimental cartridges is driven by trends. Most cartridge conglomerations have been developed in the workshop of loaders and bench shooters and most popular end products are the result of focus in trends and common misconceptions. The 556N/.223 is an amazing cartridge, although a victim of its time. I refuse to jump on the bandwagon to an experimental load when the data demonstrates clear disadvantages over the 5.56. I'll be the first to re-barrel and replace my bolt when I'm confident something comes along that out-performs the 5.56 and is readily available. I will keep my thoughts to myself to the direction the development of a new cartridge is heading, and I have discussed this in-depth with some of the industries veterans. Ultimately it boils down to understanding the cartridge as a system of complex processes, and understanding the processes to develop something better than what we currently have. It's likely trend driven loads will eventually lead to a "better" cartridge before any establishment develops something truly impressive. Our perception of an effective cartridge may be turned upside down, and involve the adoption of new materials and manufacturing processes before we see significant evolution in the infantry cartridge. I recently read feedback from an Army infantryman and he noted the effectiveness of the new 5.56 cartridge adopted by the DOD and it puts things into perspective. Given the right think-tank environment I would be interested in sharing my thoughts.

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  12. I got bored today, so I decided to do my own analysis to see if I ended up coming to the same conclusion as you. Please bear with me, this may get a little bit stream of consciousness as I write things down in the order I do them so as to not overlook or forget anything.

    ***Baseline Cartridges***

    To begin with, I thought I'd establish the properties of the two cartridges which are intended to be replaced by a GPC.

    I started off with the 7.62 NATO and immediately ran into a problem. Pressure is meant to be 50kpsi for the M80 ball (http://www.inetres.com/gp/military/infantry/rifle/762mm_ammo.html), but I need to get up to 61kpsi in Load From a Disk (LFaD) in order to equal the average velocity found here: http://www.shootingillustrated.com/index.php/28464/m80-ball-ammunition-roundup/

    After a bit of panicking, I used the Powley Computer and found that the pressure was "only" 57kspi with it. So far as I can figure, that's because LFaD measures the barrel from part way along the neck of the case to the muzzle, while the PC doesn't take that into account. So I'm henceforth going to be adjusting "effective barrel length" to how long I want the barrel length to be, rather than how long the barrel will be from the base of the cartridge to the muzzle.

    Adjusted, I can get muzzle velocity of 817 m/s with 46.6 grains of AR2208 at a pressure of just under 58.8 kpsi from an 18" barrel, or 801 m/s from a 406mm barrel at just under 58.3 kpsi (https://www.dropbox.com/s/9gxjxfjuknt5aee/7.62mm%20Baseline.png). I'm going to use the 406mm barrel length, as it seems to be fairly widely chosen as the intermediate between full length (508mm) rifle barrels and carbine (368mm) barrels.

    The M855 is meant to operate at around 55kpsi, but to match the M855PD loading (http://www.ar15.com/content/page.html?id=213), I need to push up to 58kpsi. I can get that with 24.8 grains of H335 (https://www.dropbox.com/s/awkwn4gc39lf21o/5.56mm%20baseline.png).

    https://www.dropbox.com/s/uooa7r3hpj9o49d/5.56mm%20Energy%20vs%20Range.png
    5.56mm NATO ballistics

    https://www.dropbox.com/s/9fdi0tazzd2kzep/7.62mm%20Energy%20vs%20Range.png
    7.62mm NATO ballistics

    Now, rounding down, there are 81 rounds of the M855 per kilogram, which gives us 32.61 kj/kg at 500 metres and 11.59 kj/kg at 1000.

    Rounding down as well, there are 39 rounds of the M80 per kilogram, which gives us 33.154 kj/kg at 500 metres and 12.682 kj/kg at 1000.

    These are the numbers that any GPC will need to beat.

    (to be continued due to character limits)

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  13. (continued from previous comment)

    ***Design of a Small GPC***

    My first thought was to use a 7.62x39mm necked up to 7mm, with less of a case taper for better capacity and a longer shoulder length (with a slightly shorter neck and body) to compensate for the increased angle. According to LFaD, case capacity should be 35.8 grains of water (https://www.dropbox.com/s/tx4cbp47e5zkdkx/7x39mm%20GPC.png).

    At first it was going to be a drop in replacement for the 5.56mm, not because I thought it would work, but because I wanted to see how it went, but I decided I wanted to copy the .276 Pedersen's projectile, so OAL was increased to 59.4mm.

    I used a drawing of the .276 Pedersen (http://i282.photobucket.com/albums/kk272/dundas56/garand-cartridge.jpg) to come up with a projectile roughly based on it (http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=mm.&weight_unit=grains&bullet_name=7x39mm+GPC&re_calculate=yes&diameter=7&length=30.5&nose=19.56&meplat=1.44&drive_band=7&base_diameter=5.5&angle=8.3&boat_tail=5.08&secant_radius=10.56&weight=125&density=10)

    The best I can get using this and not going over the 58.5kspi limit I've set myself is 725 m/s with 29.6 grains of AR2206 (56.5kpsi).

    https://www.dropbox.com/s/liq7wwvf3u6vf1e/7x39mm%20Energy%20vs%20Range.png
    The 7x39mm ballistics

    Now we need to work out weight. Based on some steel cased ammo 7.62x39mm ammunition (http://www.barnaul.co.nz/ammunition/selection/rifle-2/7-62-x-39-m3), and assuming that the 7.62x39mm uses 25 grains of powder (http://www.bocn.co.uk/vbforum/threads/71307-Czeck-7-62x45-VZ-52?p=145819&viewfull=1#post145819), weight should be 257.6 grains, or 16.7 grams. this gives us 59 rounds per kilogram, rounding down.

    This gives us 48.167 kj/kg at 500 metres and 20.762 kj/kg at 1000 metres.

    ...

    ...

    I did not expect that.

    Normally, I'm pretty confident in my work, but as this result wasn't something I expected and as you seem to have a better handle on this than me, would you mind double checking my work?

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  14. Hold on, correction: the 7.62 NATO should be 41.71 kj/kg at 500 metres and 15.9455 kj/kg at 1000 metres. I should ahve doubled checked that.

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  15. And I, being an idiot, used 10.91 instead of 10.81mm for the shoulder diameter, which leads to a 6m/s velocity drop. The new kj/kg at 500 metres and 1000 metres should be 47.152 and 20.608.

    Sorry about that and the succession of posts. I really need to proof read things before I post them.

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    1. Would you mind emailing me at nathanieltfitch@gmail.com, so that we can continue this conversation?

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    2. So, quickly running through this, I estimated the weight of your cartridge with a brass case to be around 17.2 grams. Keep in mind, that if you could introduce a new cartridge with a steel case, you could absolutely create steel-cased variants of 5.56 and 7.62, so it's worth keeping the case material constant for the best comparison possible.

      Given this, and your velocity figures, I am getting an figure of energy/weight at 1,000m of 12.2 J/g for 5.56 (assuming a velocity of 3,008 ft/s from a 16" barrel), 16.3 J/g for 7.62 (assuming your figure of 2,627 ft/s from a 16" barrel), and 21.6 J/g for your 7mm (126gr at 2,378 ft/s, 16" barrel). At 500m, I get 35.8 J/g for 5.56, 42.2 J/g for 7.62, and 51.7 J/g for your 7mm.

      The 7mm clearly does better than the other two, in terms of energy retained per pound. However, there are a few other things to consider, which I would rather discuss via email.

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    3. Excuse me, I am using 12.0 grams weight per cartridge for M855, and 24.2 g/cart for M80. These are taken from my own personal measurements, which have been published on this website previously.

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    4. Thanks for that brass cased estimate. It's something I definitely should have kept in mind.

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    5. Not a problem, though keep in mind that is a minimum estimate.

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    6. Why on Earth would I do something silly like use an average or maximum estimate. That could make my concept look bad :p.

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  16. Actualy the trick in creating a true intermediate cartridge that can replace both 5.56 and 7.62 Nato rounds is to try to be close to the energy of 7.62X51 in 1000yards (less energy 2-5%). The weight gain that will come by creating an inferior to mr Williams 6.5/8/800 cartridge will make the hole project feasible.

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    1. I don't really think so. See my other post discussing a lighter GPC.

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  17. I am talking 80-92 grains .224-.243 caliber with a bc of .500+ and a velocity of 2850-3050fps.
    And....... the necessity for a truly intermediate cartridge (to exist), was created by the need to have truly mixed caliber platoons .
    For example . To your typical squad , add a designated marksman with an ar-10 and replace the second m249 with a mk48 (minimi 7.62) . (Plus a 3 men element to the weapons squad with a carl gustav-We are talking about 45 men platoons that are capable of fighting anything) .
    There is no point to compare a 5.56 only armed scuad or platoon (+ 2x m240s) with one armed with even the lightest intermediate cartridge .
    Thank u for your articles and your time.

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  18. Great article, and great links in the comments. I've a day's worth of reading to do now about cartridge design.

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  19. Hi Nathaniel Fitch out of interest have you had any correspondence with Anthony Williams over this? I'd be interested in that.

    Secondly wouldn't a new GPC round make obsolete the 7.62mm and give some weight savings there?

    I often wonder would we have the two calibre system of either the .276 Pederson or .280 British rounds had been the NATO norm.

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    1. Hi Patrick, I've spoken with Tony some on forums, but he can be a bit dismissive of those who don't agree with him on certain things. I've never really gotten anywhere with him, and at times our correspondence has been a bit heated.

      I think we would still have a two-caliber system with .276 or .280. Neither are different enough than 7.62.

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  20. Great analysis, Nathaniel; much appreciated! Well-reasoned and well-thought out.
    Also appreciate the gentlemanly civility of your remarks and responses.
    Keep up the good work!

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  21. There is a better solution than status quo. A .243/6mm cartridge such as the 6mm Hagar. The end result is a round that far exceeds the 5.56 at any distance that the average rifle man can shoot at. It is in a caliber that has ample history as an effective hunting round. All the mathematics and butt and head scratching and pontificating is meaningless if the round won't perform. Such is the history of the 5.56. I could care less if it weighs less so that someone can carry more...when it doesn't stop the bad guy. The technology improvements of bullet design needs to be a key component of ammunition choice. Our troops deserve the best.

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