Thursday, January 30, 2014

All 5.56 Replacements Suck

[The title of this blog post was mandated by the 196,800 Revolutions Per Minute Marketing Department]

One of the benefits of having a system that allows me to accurately create cartridges and predict their performance is that I am able to easily make comparisons and evaluate different configurations against each other in terms of weight, size, and performance. I previously used this method to create a standalone cartridge that satisfied Tony Williams' requirements for a GPC in this post. Today, I'm going to use it to create an example cartridge that shows in part why most larger-caliber 5.56 replacements offer virtually nothing over the cartridge they would replace.

First, we whip up a 5.56 load with the Powley computer:

Naturally, I already had the 5.56 case rendered in Solidworks, based off the 5.56 case specification. The figures for projectile length are those of the SS109 projectile, a drawing of which can be found here.

Now, we'll whip up 5.56 necked up to 6.8mm. Let's use 6.8 SPC projectiles, so we'll need the same case length as 6.8 SPC (1.6864")...

We'll need to decide on a neck length for the new cartridge. 6.8 SPC has a relatively long neck, length, so I think we'll just use the neck length of 7.62x39. This'll make a fairly minor difference in performance, and I'm erring on the side of more, not less...

Here's our finished case:

Now, we have to find the case volume. To do this, we need to model the negative of the case interior, like this:

Then, once you have a solid model, we need to find the internal volume:

Alright, now we have the internal volume in grams H2O. Just divide by 1,000 and multiply by 15.43 to get the case capacity in grains H2O, which comes to 30.2 grs.

Now we need to find the length of the bullet we'll be using. Fortunately there's a handy list of these over at JBM Ballistics. The Hornady 110gr V-Max is a 6.8 SPC bullet, and will be fine for this example. It has a length of 1.045. To the Powley computer:

Alright, now we have two sets of performance figures that are about as directly comparable as we're likely to get. Now we need to run them through a ballistic calculator, the best free one of which I am aware being the one at JBM Ballistics. This is, in my experience, a very accurate calculator, that compensates even for nonlinear increases in wave drag at supersonic speeds. So we enter the projectile weight, velocity, caliber, and set the maximum range to 500 meters, range increment to 100 meters, zero to 25 meters, and sight height to 1.5". We also check "ranges in meters", and in a dropdown menu select "Energy (Joules)". Next, we just need to find some accurate ballistic coefficient figures. Fortunately, I have an Excel spreadsheet of such figures on my website, which just so happens to include the value for the .277" Hornady 110gr V-Max. For 5.56, we'll derive a ballistic coefficient value with the same form factor as the V-Max, in the interests of keeping everything fair. We get a value of .159 G7, very close to SS109's .158.

Now we can get a good estimate of the performance of these two cartridges.


...and our 6.8/5.56 cartridge with the 110gr V-Max:

Ahah! See? The 6.8mm provides a full 18% more energy at 500m than 5.56! I knew it! Larger calibers really are better!

...Wait, is that the whole picture? After all, isn't the 6.8mm heavier? What good is a military round that produces somewhat more energy per shot if it doesn't end up saving you any weight? Well, to find out exactly how much heavier our 6.8mm is, we need to go back to Solidworks, and find out how heavy the cases of each round are:

We can see from this that the weight of the cases is very comparable. To get the actual weight of each case, we divide the highlighted figure by 1,000, and multiply by 8.4 (approximately the density of drawing brass). For 5.56, we get 6.372 g, and for our 6.8/5.56, we get 6.298 g. Next, we need to find the weight of the powder charge for each. Well, hey, we already know this, it's right there in the Powley computer results. Converting to grams by dividing by 15.43, we get 1.458 g for 5.56, and 1.283 g for our 6.8. Hey! So far the 6.8mm is winning!

There are two more components left. One is the primer. I happen to know that small rifle primers weigh about .25 g, and both are cartridges use SRPs, so we'll add .25 g for both.

The last component is the bullet. We know the bullet weights already; 62gr (4.02g) for 5.56 and 110gr (7.13g) for our 6.8. This is where the 6.8's minor lead in terms of weight gets completely shattered.

Adding it all up, we get a total round weight of 12.1 g for the comparative 5.56 and 15.0 g for our 6.8mm cartridge.

Now, if we take the energy at 500m and divide it by the weight of the cartridge, we can truly begin to compare these two cartridges. For 5.56, we get a value of 35.5 J/g@500m. For our 6.8mm, we get a value of... 33.7 J/g@500m. That's right, in terms of energy retained at range for every gram carried, the much-loathed 5.56mm actually comes out 5% better than its 6.8mm counterpart. Who'd have thought?

Keep in mind that 5.56 also produces a vastly superior trajectory to the 6.8mm cartridge, having a whopping 81% less drop, 19% less wind drift, and 25% better time of flight, at 500m. Further, remember the principles of wounding when looking at these two cartridges. Smaller projectiles, all things being equal, tumble sooner than larger ones. Faster projectiles tend to fragment more dramatically than slower ones. Not only is the 6.8mm projectile much larger, it is also much slower. Therefore, we cannot expect it to exhibit fragmentation except at the absolute closest range, and even then, it will not do so very dramatically. If it tumbles, it will do so later than comparable 5.56 caliber projectiles, and may not deposit very much energy (link begins a download) within a human target.

Is this comparison perfect? No, it makes several compromises in exactness for the sake of clarity. Does it reflect every larger-caliber 5.56 challenger out there? No, but it does give the reader an idea of what effect increasing the caliber and projectile mass and reducing the velocity has on its performance and the overall weight of the cartridge (or, taken in the inverse, one might ask which produces better ballistics, 6.8 SPC or 6.8 SPC necked down to .22 caliber?). It is unfortunate that this material is too technical to be introductory, as it is so fundamental to understanding rifle cartridge design. Much time and effort that has been wasted might not have been if the designers of these medium-caliber intermediate cartridges had understood these principles.

A closing note: This comparison humors the assumptions of many opponents of 5.56 that the amount of energy a cartridge produces per pound carried is the sum worth of that cartridge in combat. I dispelled this notion of "stowed kills" in another post. One should not confuse my demonstrating that 5.56 meets and beats many of these proposals on their own terms with my validating that method of evaluating cartridge performance for infantry rifles.

Wednesday, January 22, 2014

Incitatus (The Remarkable Reliability Of An AR-15)

Much is said of the supposed unreliability of rifles in the AR-15 family. For those who know the rifle only from articles posted to the internet, it's obvious that the weapon is a massive failure. It's fragile, jams constantly, "shits where it eats", and chokes up completely when exposed to any sort of extreme environment.

Well, that's what I believed once, anyway. The reality of the matter is that the AR-15 is a tremendously well designed, mature, and very reliable family of weapons. My first exposure to how very wrong I was about the AR-15 came when, frustrated by substandard alternatives, I broke down and bought the closest thing to a military M4 I could get - a Colt 6920, which I named "Incitatus" as a jab at Colt's logo, its high price ($1400 at the time), and what I felt must surely be my own growing insanity.

I shot the weapon and kept shooting it. From the beginning I fired imported Russian steel cased ammunition through it almost exclusively, almost begging it to choke, so that I would be proven right once and for all. I kept pushing it further, firing it in more and more testing conditions, in sub-zero temperatures, covered in dirt, through wind and rain and through some very nasty dust storms in New Mexico. I almost never cleaned it, and it usually saw at least a thousand rounds from the last time before I lubed it. It didn't matter. Nothing stopped it. In the span of about a year, I fired nearly 5,000 rounds of (mostly) steel-cased ammunition through the rifle, and once it failed to lock back on an empty magzine. I never had another malfunction of any kind.

After all this, my old opinions of the rifle were thoroughly destroyed. So, this was the rifle the troops were using. It was a damned good one. It was almost freakishly light in comparison to its stablemates, extremely reliable, and accurate enough for me to pluck the highest score three times in a row at Appleseed events.

I stretched its legs, too. Far from the Internet wisdom that says the AR-15 is "only a 300 yard weapon", I consistently made hits with that same crappy Russian ammo out to 400, 600, and finally 900 yards before it began to struggle. What was all this I heard about the M4 being unsuitable for the fighting in Afghanistan? With the TA01NSN ACOG I'd bought a couple years earlier secondhand, Incitatus had very long legs, indeed.

Later, upon hearing me gush a bit over my rifle, a classmate of mine who had served with the Army in Iraq and had a bad experience with the M16, challenged me to a bet: I would choose the worst of his bringback magazines, and try to fire a full 30 rounds through my Colt. If it didn't malfunction, he'd buy me a pack of beer. We chose a particularly nasty example that had bent feed lips, was more of a parallelogram than a box, and had broken all the welds along the spine and been re-welded - poorly. It was time for a range trip.

Out to the wooded foothills of Colorado, we went. After a long dirt trail, barely traversed by my friend's '86 silver Toyota Camry, we stopped, and broke out our rifles and handguns, and began shooting. I gave my friends a background on the bet, and my girlfriend fired up my handheld Casio camera to record proof.

The rifle and magazine ran like a champ... Through a total of about 150 rounds, in fact. By the end of the range day, I never got the magazine to cause the rifle to malfunction.

I've read a lot of forum posts, blog posts, and magazine articles that bash the AR-15. For a while, I was convinced they all couldn't be wrong. I've since learned, not only how reliable an AR-15 can be even through neglect and tough conditions, but also not to jump to conclusions based only on an opinion I read somewhere.

Having said that, I'll leave you with a few blog posts and online articles that buck this trend, and talk about the virtues of the AR-15:

A blog post by Andrew Touhy on how cleaning your AR-15 is a waste of time.

An article by Mike Pannone on the reliability of the AR-15 platform, the "shits where it eats" myth, and problems with maintenance in the service.

Weaponsman weighs in on Congressional criticism of the M4.

A report on early M16 reliability in Vietnam, from Weaponsman.

Weaponsman on why the SCAR-L was not adopted.

The forward assist on the M16/M4 is useless, says Weaponsman.

M16 improvements from 1968, from Weaponsman.

M4 improvements, from Weaponsman.

US small arms reliability, from Weaponsman.

Friday, January 10, 2014

The New Caliber Mafia

First there was the Bomber Mafia in the 1930s, then there was the Fighter Mafia in the 1970s. Now, in the oughts and teens of this century, we have a Caliber Mafia. How droll!

On every corner of the military-related internet, it seems, one will be assailed by true believers who assure you that the 5.56mm and other small caliber high velocity cartridges are the products of a failed concept and must be replaced by something else, which inevitably must be much heavier and greater in caliber.

I've already addressed the technical concerns of why these calibers cannot replace 5.56 NATO and other SCHV cartridges in service. This post will - unlike the last which addressed Mr. William's article specifically - be a more general response to the Caliber Mafia, incorporating a number of ideas mentioned in the Tanknet thread on that post. Each section will address common arguments made by mafiosos, both those critical of 5.56mm and in favor of larger caliber cartridges.

I. Higher performance is attainable using modern powders
Ex. The 6.5mm Arisaka mentioned in TCA generated over 2,600 J from a 50mm-long case about the same diameter as the Grendel's over a century ago, without the benefit of modern powders, so the GPC should be usefully smaller as well as lighter (the Arisaka used a 9g spitzer bullet).
This argument is often used in concert with predictions of extravagant performance of a pet cartridge. For a start, propellant technology has been fairly stagnant for the past half a century. In addition, the energy density of the propellants has not improved much. Further, because the energy of the projectile is produced by the pressure curve of the propellant, gains basically cannot be made without slower burning propellants, and that only works up to a point, limited in military use by muzzle flash and muzzle thrust (the hypothetically ideal "plateau" burn curve can only be achieved by introducing more propellant into the combustion chamber after ignition has begun), as well as the practicality of loading large quantities of such ammunition quickly. As an example of how an improvement in velocity implies a rise in pressure, when nitrocellulose powders were introduced in the late 19th Century, the performance of cartridges of a given size greatly increased, but so too did the average peak pressure. Expecting significant gains in performance against historical cartridges without increasing peak pressure will thus leave one disappointed.

The cited quote from Mr. Williams is a particularly good example of this, as he simply ignores several important factors in making this statement. For instance, the 6.5x50SR Arisaka  produced between 2,500-2,600 J with an 800mm barrel, which is 60% longer than the barrel length prescribed for the GPC (20"/500mm). Thus, Mr. Williams makes a mistake in assuming that my statement at the end of this post was a predictive one; it was, rather, observational: It is highly unlikely that a cartridge will produce performance greatly superior to the 6.5 Arisaka from a significantly smaller cartridge if it is constrained to 20" barrels, even if it is using current propellants and pressure levels.

II. 5.56mm weapons are dead weight in long range engagements
Ex. The weight advantages of 5.56mm weapons and ammunition become irrelevant when a substantial proportion of small-arms engagements take place beyond their effective range - they then become useless dead weight.
Riflemen are dead weight in long range engagements. In fact, the last time this wasn't true, rifles came issued with volley sights and riflemen massed up in large formations, doing their best impression of a couple of mortar teams. The idea that a new caliber will change this seems a bit optimistic to me. There is a conversation worth having about the the increase in effective range of infantrymen from 300 to 500m brought about by the introduction of durable optical sights. However, one should also consider that 5.56 was originally intended to fulfill a 500m requirement, which current-issue M855 ammunition improves upon considerably. A tentative conclusion that 5.56 is well-suited to 500m combat is thus reasonable.

III. Terminal ballistics is too complex for science
Ex. You won't get scientific proof, because there are too many variables, as we all know: most importantly, the exact path of and damage inflicted by the bullet, plus the physical and mental state of the target. Laboratory tests cannot replicate these factors.
So help me god I've actually had to address this argument. Scientific inquiry is capable of producing far, far more complex deductions than those asked of a simple ballistics test. Laboratory experiments have been conducted that boggle the mind in their precision and control for numerous variables. To claim that a quantification of terminal effectiveness is "beyond science" is simply ludicrous. For posterity, I will re-post my suggestion for an experiment here:
Shooting at live, restrained pigs connected to sphygmomanometers, heat rate monitors, ECG machines, and EEGs, counting only precise shots accurate to within a tolerance (determined by a medical professional) on a target area of the body (this could be the heart, brain, an artery, or lung, etc), a number of different rounds of ammunition, controlling for a variable (e.g., projectile weight or muzzle energy) are fired. The results from the devices are then measured and evaluated by medical professional of that specialty as well as veterinarians. Rinse and repeat for each variable you need to isolate.

IV. Larger caliber cartridges are more effective, this expert doctor says so
Ex. Dr. Grabinsky talks about rifle wounding mechanisms and effects. Incidence through torsos, especially longitudinally are usually very lethal. The reason being a longitudinal injury has more tissue for the bullet to begin yawing. If there is no yaw, then the major wounding effect (aside from striking bone) is going to be the wound cavity, permanent and temporary.
So, a larger bullet means larger wound cavity, temporary and permanent aka crush injury and stretch injury.
This follows from a misunderstanding of the term "wounding" as used in the medical sense, and a conflation of higher energy cartridges of 7.62mm caliber (such as .30-06) with lower energy cartridges of the same caliber (such as .30 Carbine) on the part of both doctors and the readers of their research. The difference between wounding as is relevant to the medical profession and incapacitation is explained in this document, from the Ballistics Research Laboratory.

A word about Dr. Martin Fackler: His research is often used to support arguments that only the permanent cavity of a wound channel matters for incapacitation, or that it matters the most. He himself does not say this anywhere, so far as I know. Fackler, instead, addresses (among other things) the notion that tissue around the wound in high velocity gunshot wounds (which look quite nasty, indeed) needs to be excised for treatment. His studies show that the tissue damaged by the temporary cavity will recover, and that energy deposition has no effect on treatment, saying nothing about incapacitation. Doctors, obviously, are concerned with the former, while small arms designers are largely concerned with the latter.

V. 5.56's terminal performance is unreliable; a larger caliber projectile will produce more consistent results

This is an argument I see implied almost any time the subject is brought up. Evidence of inconsistent terminal effectiveness in 5.56 is provided, and thus "we need a new, larger caliber" to fix it.

Evidence suggests that this will only make the problem worse. If one is limited to Hague-compliant projectiles, then tumbling and fragmentation will be your primary vectors for terminal effectiveness. Not only do larger calibers (all things being equal) tumble less readily than smaller ones, they also often don't have enough velocity to fragment consistently. 6.8 SPC FMJBT bullets, for instance, hardly fragment at all, even at very close range. Dr. Fackler even noted that 5.45x39 7N6 projectiles tended to upset sooner than 7.62x39 projectiles of similar construction. The implied notion that, despite this, an additional few hundredths of a square inch frontal area will drastically improve effectiveness leaves me a bit skeptical.

VI. M855 fired from an M4 fragments out to only 50m
Ex. After 200 m M855 doesn't fragment if fired from 20 in barrel (M16). From M4, it doesn't fragment at distances greater than some 50 m due to a lower mv that drops to the critical velocity at shorter distance. Shorter barrels are even worse.
The response to this is very nuanced and complex, and thus wholly unsuitable for the type of soundbyte-based debate that occurs on internet forums. While the fragmentation of small arms projectiles does change with the velocity at which they impact, use of the term "fragmentation threshold" can be misleading. If a projectile is fired at just below the fragmentation threshold, it performs much the same as if it is fired just above. The fragmentation threshold thus does not denote a drastic transition in performance of conventional jacketed small arms projectiles at a certain impact velocity. It is useful only in eyeballing how the projectile performs at different speeds, as at speeds below the threshold, no fragmentation occurs, while at speeds above it, fragmentation occurs in progressively more severe fashions. Only at very high velocities (typically over 2,900 ft/s, depending on jacket construction) does the familiar "confetti" fragmentation pattern occur. The reader should also keep in mind that fragmentation depends on many factors, the most important of which, besides impact velocity, is the construction of the bullet. Some materials fragment at very low velocities, while others may fragment only at velocities above that which is practical for nitrocellulose propellants. The figures used here are a "rule of thumb" for jacketed, lead-cored bullets, but even within that scope they can differ significantly from reality.

I am going to try to make this as brief as I can, but this section of my response is fairly technical and involved, as it covers a "worst case" scenario for velocity at range for the M4 Carbine in some detail. A military barrel is considered to be worn out if it experiences a velocity loss of 200 ft/s or more vs. a new barrel. The standard set in MIL-C-63989C defines the average velocity of M855 from the M16A2 to be 3,000 ft/s (+/-40) at 78 feet from the rifle, which equals an average muzzle velocity of 3,081 ft/s. From the 14.5" barrel of the M4 Carbine, we can expect no more than a 9.6% reduction in velocity,* for an average muzzle velocity of 2,811 ft/s. That gives us a muzzle velocity from our unserviceable barrel of 2,611 ft/s.

Now, we can plug this figure into a ballistic calculator and see if our commenter is right. I am using the above velocity (2,611 ft/s), a ballistic coefficient for M855 of .151, a zero range of 25m, a maximum range of 500m, a range increment of 1m, and a minimum fragmentation velocity of 2,140 ft/s.** The result is that the bullet reaches minimum fragmentation velocity at 154 meters. If a threshold of 2,300 ft/s is used (which I've seen quoted a few times), then it reaches that velocity at 101 meters. Only if a threshold of 2,500 ft/s is used does the fragmentation range drop below 50 m. This is not the minimum threshold of fragmentation, but the upper bound minimum velocity at which the jacket may split along the cannelure.

Keep in mind, an M4 that clocks velocities this low is considered unserviceable and should be removed from service and fitted for a new barrel. If a more reasonable velocity of 2,970 ft/s* is used, the M4 Carbine stays above the 2,140 ft/s until 260 m, and above 2,300 ft/s until 207 m. Even if a threshold of 2,500 ft/s is used, a muzzle velocity of 2,970 ft/s gives a fragmentation range for the M4 of 143 m.

*I don't think SADEF's figures are representative enough to be used outside of the scope of their experiment. The test was interesting, but I don't really think 9.6% velocity reduction is an accurate figure for the velocity loss going from 20" to 14.5" barrels (it results in approximately 50 ft/s lost per inch!). However, I'm using it here as a "worst case" example. Field Manual 3-22.9 provides a more reasonable muzzle velocity figure for the M4 of 2,970 ft/s, which is a loss of about 25 ft/s per inch from the M16's nominal muzzle velocity of 3,100 ft/s.

**I use a minimum fragmentation velocity of 2,140 ft/s, which is close to the lowest velocity at which fragments will come off of the bullet (usually shed from the lead core). The picture used as an example of this is of M193, but M855 performs basically the same way at comparable velocities, having the same jacket thickness.

Note: There are a lot of different figures thrown around for the muzzle velocity of the M4, both in this section of this post and elsewhere on the Internet. While it may be desirable to keep a nominal muzzle velocity figure for a given rifle and ammunition on hand, one must remember that many factors affect the muzzle velocity of a rifle, beyond the type of ammunition fired and the barrel length of the gun. Such factors include - but are not limited to - the temperature of the ammunition just before firing, the profile and contour of the bullet, the shape and dimensions of the rifling, and the wear on the barrel. I took considerable effort to use the lowest velocity figures that seemed reasonable to me in every instance, to try to weigh the examination in favor of the idea that the M4 has a critically short fragmentation range. For example, FM 3-22.9 gives the muzzle velocity of the M16A2 as being 3,100 ft/s, not 3,081 ft/s (calculated from the specification in MIL-C-63989C). Even so, I was only able to achieve a fragmentation range of 50m by using a very high fragmentation "threshold" of 2,500 ft/s.

VII. 5.56 relies on fragmentation to incapacitate

M855 and M193, like all military rifle projectiles, rely on energy deposition to incapacitate targets. This is why ballistic gelatin is such a good indicator of performance, especially if high speed video footage is taken of the shot. At high velocities, 5.56mm FMJs will fragment, which can cause very grievous wounds indeed, but even if they do not fragment they will still tumble and deposit energy. Further, the single biggest factor in incapacitation is shot placement. It is unlikely that any 5.56mm projectile will incapacitate the target with a shot to an extremity, but the same is also true of full-caliber 7.62mm projectiles, as well. As noted before, smaller-caliber projectiles will tumble earlier than larger ones, all things being equal, and thus will tend to deposit a greater percentage of their energy into the target.

VIII. 5.56 produces only 2,700 ft/s from the M4

This is based on a SADEF Journal article using a nonstandard barrel, with ammunition chilled before firing for temperature consistency. It is not applicable to M4s with good condition barrels used in temperate conditions, which typically have muzzle velocities about 200 ft/s higher or more.

IX. Permanent cavity is the most important factor in incapacitation

This Ballistics Research Laboratory paper disagrees.

X. "Stowed kills"
Ex. If it takes 3 hits to put down a Taliban Fighter, are you saving weight over a cartridge that takes one hit to do so? I think not.
This is the way I have seen the term "stowed kills" used in small arms circles (it is used in a completely different way when talking about AFVs): the speaker describes (explicitly or implicitly) some sort of modifying coefficient to ammunition. e.g., it has been argued that 6.8 SPC is 3.5 times effective as 5.56, and weighs 40% more, therefore it is overall 2.5 times as efficient as 5.56.

I argue that this is nonsense. The reason being that very few rounds fired from infantry rifles ever hit their intended targets. Most infantrymen who've seen combat have not shot directly at another person very many times at all. I would hazard a guess that the number of enemies hit by ammunition fired from rifles in combat per combat veteran rifleman is decidedly in the single digits, and may even be less than one (I'm being extremely generous here, given figures from past wars). The number of rounds expended per combat veteran rifleman, however is assuredly much higher, probably in the triple digits bare minimum.

Let's go with some ballpark figures. Say the average combat veteran rifleman expends 5,000 rounds of ammunition over his combined tours of duty, and hits and at least wounds 2 enemies in that time. That means, if he was using a 5.56mm rifle, he would have expended 60 kilograms worth of ammunition, only a few tens of grams of which had any physical affect on the target at all. Nearly 5,000 rounds he expended, minus the ones fired that hit their targets, produced exactly zero kills. Only a handful of cartridges were directly responsible for taking the enemy out of action, so even if a more poorly-performing caliber is used which requires a soldier to fire many more rounds to incapacitate a target, that fraction of the total rounds expended over that soldier's tours in terms of weight is still very small. This will be true regardless of whether the cartridge is 5.56mm, 7.62mm, or anything else. Therefore, "stowed kills" as it is typically used in the context of infantry rifles, is not a useful metric.

XI. Twist rate has no effect on the terminal effectiveness of 5.56mm
Ex. Fact: Flesh is as much as 1000 times denser than air and will cause a bullet to lose stability almost instantly. For M193 and M855 ammo, this typically occurs after 3-5 inches of flesh penetration, though this can vary. In order to spin the bullet fast enough to be stable in flesh, the barrel twist would have to be on the order of 1 twist every 0.012 inches, which would look like the barrel had been threaded instead of rifled.
This unfortunately results from a misunderstanding of how a bullet travels in flight. It is true that a bullet spun by rifling cannot hope to remain stable for long in a mostly-water medium like tissue or ballistic gelatin. However, this is not the only factor in how and at what point in its travel the bullet will tumble.

The twist rate of the barrel helps determines the stability of the projectile through media, in this case air. A tighter twist rate will better stabilize the projectile, reducing the precession of the bullet (the degree to which it deviates axially from the flight path). It is this reduced angular deviation that can cause through-and-through wounds, not the bullet being stable through flesh. In other words, a bullet stabilized by a 1-in-7 twist rate barrel may hit the target at a shallower angle and thus yaw later than one stabilized by a 1-in-9 twist rate barrel. I highly suspect this is why you will be hard pressed to find a gel test video online of M855 being fired from a 1-in-9 twist rate barrel and failing to upset within about the first 5".

Somewhat paradoxically, this tight twist rate should give M855 exceptionally consistent long-range terminal effectiveness. The same excellent stabilization that minimized precession also ties the bullet more closely to its original orientation through its flight. That means that at long range the bullet is flying through the air at an upward angle relative to the arc of its flight. If it hits a target at this angle, it should upset readily and tumble within the first few inches of tissue.

EDIT (3/23/2014): It seems I may be wrong about this. A closer reading of Small Caliber Lethality shows that in testing longer projectiles which would have been less well stabilized than M855 from 1/7 twist barrels, they found virtually no difference in fleet yaw from M855 and any other caliber tested, including M80. The reason for this erratic performance is that within 50m the projectiles have not yet settled into stable precession caused by their rifling (something I am wholly unqualified to describe). More on this can be found here. While the explanation of twist rate's effect on lethality is still incomplete, it seems my theory wasn't quite on the mark, either. I am leaving the incorrect explanation up as a record of my mistake.

XII. The GPC concept was formulated in light of experiences in Afghanistan

Mr. Williams will freely admit he first came up with the concept in the early '70s, which inclines me to believe that he has come up with requirements to suit his concept, and not the other way around. Regardless of the merit (or lack thereof) of the GPC idea, it is clear that it was not developed in light of recent experiences overseas.

XIII. Soldiers are unhappy with the performance of 5.56

Soldiers may be unhappy with many things, but there's no reason to believe the performance of 5.56 has not been satisfactory. Several notable combat veterans have commented on their satisfaction with the cartridge, noting especially its light weight. Video evidence corroborates its effectiveness in skilled hands, even at long range.* Given this, it is somewhat strange to be met with constant cries of "the soldiers don't want it!" which cannot then be corroborated with actual sources, anecdotal or otherwise. Often "after action reports" that support the idea that 5.56 is inadequate are implied to exist, but when asked to produce these reports, the speaker cannot supply anything of the sort.

*The ammunition used here is the 77gr Mk. 262 special purpose ammunition, not M855. At 800m, neither projectile fragments, but both tumble readily.

EDIT (3/6/2014) XIV: 5.56mm produces inadequate suppression effect for an infantry rifle cartridge

Mr. Williams has become quick to claim that 5.56mm produces inadequate suppressive effect - by way of a small sonic boom - and that a 6.5mm weapon would do much better.

What this ignores is how sonic booms are actually generated, and what aspects of a radially symmetrical body enhance or mute the boom. Fundamentally, the boom is created by the pressure wave, and is closely related to how drag operates on the body itself - which is more a function of shape than size. I'm don't have an Aero/Astro degree, so I can't really go into detail here, but it's important to note that while larger bodies do create larger sonic booms, a .264" bullet from a GPC is simply not bigger enough than a .224" bullet from a 5.56mm round to make an appreciably larger sonic boom. Further, a center piece of the GPC concept is the use of elongated, low drag bullets. These bullets, for exactly the same reason that they retain energy well, will produce inferior sonic booms to those that have inferior ballistic shape. The energy for the sonic boom can only come from one place: The projectile itself as it moves through the air. Therefore, lower drag bullets will necessarily produce smaller, less audible sonic booms than higher drag ones; they simply do not expend as much energy in flight.

One way that the GPC could potentially produce louder sonic booms is that it retains velocity better, which is a major component in sonic boom generation. However, the GPC only exceeds the velocity of 5.56mm (when fired from comparable barrel lengths) at 250m, and 5.56mm only becomes subsonic at 700m, meaning the gains in this area may well be negligible.

Overall, the biggest problem with Mr. Williams' theory about the sonic boom-producing ability of the GPC is that he has provided no direct evidence for it. Further, the overwhelmingly most important factor in the suppression of enemies is how close the bullets impact to the target. The round that an infantryman will miss less badly with is the round he can carry and shoot without tiring for the longest, given an adequately flat trajectory.

In Sum:

There is not sufficient weight to the arguments of the Caliber Mafia to compel me to take their ideas seriously, much less for any military to actually implement any of their proposed calibers as standard issue. While the 5.56mm caliber and the two-caliber system may not be ideal, it is sufficient to meet current needs, so far as this author can tell. What does the future hold? Who knows? Whatever the next infantry rifle cartridge is, however, it is unlikely to produce significantly less velocity or fire projectiles any larger in caliber than 5.56mm.