Wednesday, November 20, 2013

Steel-Cored Armor Piercing Loads for 5.56mm

As body armor improves and as the presence of lightly armored vehicles increases on the battlefield, it is reasonable to expect that inexpensive steel cored armor piercing rounds will become more desirable. Currently, there are 5 steel cored armor piercing loads widely available, in all calibers: .30-06 M2, 7.62x51 M61, 7.62x54R BP, 5.45x39 7N22, and 5.8x42 DBP-10.

These loads provide a good starting point for designing a new steel-cored AP round for the 5.56mm.To gain some idea of the effectiveness of each projectile's core, we will examine their respective length-to-diameter ratios, gleaned from images of the sectioned bullets. Unfortunately, I was unable to find any reference material for the Russian 7.62mm BP projectile. The rest are as follows:

M2: Between 4.1 and 4.6 calibers, depending on sample 
M61: Between 3.3 and 3.7 calibers, depending on sample 
7N22: 5.2 calibers
DBP-10: 4.9 calibers

Because the M2 and M61 cores have such uncompetitive L/D ratios, we will forgo scaling those projectiles down, and instead focus on analogues to the 7N22 and DBP-10.

Scaling the 7N22 projectile to .224" caliber, we get a weight of 59.9 grs and a length of 1.020". Doing the same for the DBP-10, we get a weight of 60.7 grs and a length of 1.015. So far, both bullets seem very comparable, with the 7N22 projectile having a somewhat higher ballistic coefficient.

Bullet seating will be calculated both for the intended depth of the original 7N22 ammunition, and to fit within 5.56mm's OAL, respectively. The former produces a figure of 2.488" overall length.

The 5.8mm caliber has two versions: one intended for rifles, and one intended for DMRs and machine guns. The former has an overall length of 58mm, while the latter has an overall length of 61mm. The case length is 42mm, giving the each bullet 16mm and 19mm of exposure, respectively. We will use these figures, adjusted for caliber to 15.19mm (.598") and 18.03mm (.710"), as well as the original figure for 5.56mm of 12.70mm (.500") to give us overall lengths for each.

Utilizing the Powley Computer, we can determine performance for all five configurations:

Fig. 1: 5.56mm with scaled down 7N22 bullet and the same relative seating depth as 5.45mm

Fig. 2: 5.56mm with scaled down 7N22 bullet

Fig. 3: 5.56mm with scaled down DBP-10 bullet and the same relative seating depth as 5.8mm heavy ball

Fig. 4: 5.56mm with scaled down DBP-10 bullet and the same relative seating depth as 5.8mm light ball

Fig. 5: 5.56mm with scaled down DBP-10 bullet

Fig. 6: A hypothetical 60gr steel-cored 5.56mm AP load
As we can see, performance is very similar for all types. The ideal configuration might be found in an adapted DBP-10 projectile - perhaps with a proportionally larger core - designed to fit within the existing 2.26" OAL of 5.56mm. This would give comparable performance to the DBP-10 within 500m, and cause minimal logistical disruption. The projectile could have reverse-drawn jacket with closed tip giving it a good ballistic coefficient and excellent accuracy, while conforming to international treaties. The finished product might look something like the render in figure 6.

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:

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
10/11/13 18:34, JBM/jbmtraj-5.1.cgi

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
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.