By Andrew Betts
If you spend enough time at an outdoor range, especially on the weekdays when the retirees are there in force, you are certain to hear someone opine that they prefer the M14 to the AR/M16/M4 because it “turns cover into concealment”. This is usually in conjunction with their opinion that the DoD made a terrible error in moving to the 5.56x45mm, rather than the much more manly 7.62x51mm. No one can claim that the 7.62mm NATO does not have more power. The cartridge contains significantly more powder and it launches a heavier bullet at only moderately lower velocity. Is that extra power actually useful for penetrating cover, though? Does it really “turn cover into concealment”?
To answer that, we took a look at a few real world objects of varying composition. The question is not whether the 7.62mm penetrates more deeply than the 5.56mm. It is widely known that 7.62mm will penetrate more deeply in some materials such as wood, while 5.56mm can often penetrate steel plate at close range better. M193 55 gr FMJ can even defeat Level III armor plates that are rated for multiple 7.62x51mm M80 147 gr FMJ (https://youtu.be/QrWtgyFQ8LU). The claim that the old guys are making is that the M14 can kill a man who is hiding behind an object that would stop a 5.56mm. In other words, does a small difference in penetration depth really translate to a difference in whether a specific object will act as cover or not? If the cartridges are compared in terms of go/no-go, will the M14 really “turn cover into concealment”?
For the first test, we will consider concrete barriers. There are a variety of concrete walls, block walls, and other concrete barriers in the urban landscape that a person might take cover behind. The concrete varies somewhat in the ratios of the ingredients but all are composed of cement, sand, and sometimes larger aggregate. Regardless of the recipe, concrete has high compressive strength and low tensile strength. That means that it works very well for applications such as load bearing walls, but not so well for a second story floor. It resists being crushed but when bent, it cracks easily. That also means that it works pretty well to stop a bullet, but it is destroyed in the process. We tested two kinds of concrete. The first is a concrete block common to privacy fences, with lots of small aggregate and air voids.
The second is a concrete paver. While not as sturdy as a poured concrete wall, the paver is made from mostly cement and sand, with little aggregate and no air voids.
In both tests, neither round was completely stopped by the concrete barrier. While the 7.62mm did look more impressive, the 5.56mm also made it through and neither cartridge seemed to retain much ability to wound on the other side of the wall. That is to say, both would likely cause a painful wound but neither were likely to penetrate deeply enough to have a high probability of causing incapacitation. A bad guy on the other side of either of those barriers would have an awfully bad day to be sure, but he would likely have the opportunity to make your own day much shorter. To sum up, it is a very close race with little practical difference between the two cartridges.
Of course, an 8” thick, poured concrete wall with rebar reinforcement is likely to stop both rounds cold, but it is also outside our ability to test. There are almost infinite variations on the thickness and composition of concrete structures and some will certainly stop both cartridges while others will not stop either cartridge, as seen in the above tests. It would take substantial resources to conclusively identify exactly what sort of barriers could be penetrated by which cartridge and at what distance. For our more general and limited testing, the conclusion is that both cartridges can penetrate some concrete barriers. There may very well be a special Goldilocks barrier that is just thick enough to stop the 5.56mm but not the 7.62mm. From what we can see of this testing, it seems likely that such a barrier would also bleed so much energy from the 7.62mm as to render it nearly harmless, though. Both cartridges failed to fully penetrate a single water jug in this test so if the thickness of the concrete were increased to that magical point where 5.56mm was stopped but 7.62mm passed through, the 7.62mm would be even less energetic than was seen in this testing, which means a very minor wound.
Next, we will consider one of the few components on a motor vehicle that actually has a good chance of stopping a bullet: a brake rotor. Other than the drive train, the brake rotors (or drums) are one of a very few places where there is actually enough thick metal to have a reasonably good chance of stopping a bullet. Frame rails will usually stop handgun rounds but are unlikely to stop any rifle round and it is common knowledge that the body does next to nothing to stop a bullet. Conversely, the engine and transmission should stop nearly any man portable weapon short of an AT-4. Will the brake rotor be just thick enough to stop one cartridge, but not the other?
In this case, several rounds of both the 5.56mm and the 7.62mm were stopped. It is true that the 7.62mm looked to be a bit closer to getting through, based on the slight cracks on the back side of the disc, but the bottom line is that a person hiding behind that object would not have acquired any extra face holes from either cartridge.
Wood is one of the materials which 7.62mm is said to penetrate much more deeply than 5.56mm so we compared the two cartridges’ ability to penetrate a modest sized log.
On the one hand, the 7.62mm penetrated almost twice as much wood as the 5.56mm. On the other hand, both were stopped and you would need to find a log that was more than 2 ½” thick but less than 4” thick to be able to stop the 5.56mm but not the 7.62mm. Aside from the obvious problem that few people would consider a 4” stick to be “cover”, the difference here underscores something we have long suspected. It is true that 7.62mm can penetrate more deeply, but the difference is unlikely to make any substantive real world difference. That is to say, there are very few objects that are just thick enough to stop a 5.56mm but not thick enough to stop a 7.62mm. Most objects are either thick enough to stop both or thin enough to stop neither.
We did find one material that was soft enough to underscore the difference in a very definitive way: water. This is a test using a 55 gallon plastic drum filled with water as the barrier.
Finally, here is an object that very clearly stopped one bullet but not the other. If your target is taking cover behind a 55 gallon plastic drum full of water, 7.62x51mm can punch through it, while 5.56mm will probably be stopped. In the high speed video, it seems that the 7.62mm was not really moving along that quickly after passing through the barrel, though. It is possible that it would not be capable of doing much wounding after getting through the barrel, but we did not test for that, so the nod has to go to the 7.62mm for getting through.
It is also worth noting here that projectile construction could make a significant difference in any of these tests. If the rounds were changed to bonded soft points, it is possible that both rounds would have made it through the water. If the 7.62mm were a Hornady 155 gr AMAX, it is unlikely it would get through the barrel. There are a wide variety of bullet weights and designs available for both cartridges and some of them will substantially change the performance on these objects. We chose M80 and M855 because they are the commonly issued FMJ ammunition for their respective rifles. We chose a 16” barreled AR15 because it is a good compromise length and we did not have the time to test 11.5”, 14.5”, and 20” barrels. We also did not test at greater distance, where the 7.62mm is likely to have a larger advantage because hauling the test materials 200 yards down range is difficult, bothersome, and disruptive to other shooters. There are a variety of conditions that were not tested and those conditions could give more of an edge to one or the other cartridge.
Overall, most of the tests showed very little difference between the two cartridges. In every test but the water barrel, either both penetrated the test object or both were stopped. Ultimately, it does not appear that there is any evidence to support the unilateral claim that 7.62x51mm “turns cover into concealment”. There may be some very specific circumstances where this is true, but they appear to be the exception, rather than the rule. To be sure, this concept deserves quite a lot more testing. It would be nice to see the differences at range and through a variety of other materials such as live wood and poured concrete. Some day we may continue testing. It seems that the M14 is likely to develop a real, substantive advantage as range
increases because the greater mass and higher ballistic coefficient can carry more energy further down range. On the other hand, this sort of testing only compares a single round of one cartridge to a single round of the other but 7.62mm weighs twice as much and that means a person is likely to have twice as much 5.56mm. In that light, one round of 5.56mm may be just about as good as one round of 7.62mm but two rounds of 5.56mm are far better than one round of 7.62 in nearly any circumstance. The real take-away here is that nothing in the world of firearms and projectiles is nearly as simple as “A is better than B” and it appears that the statement “The M14 turns cover into concealment,” is more often false than it is true.
Related further reading of 762 penetration