Bear with me friends, this week’s gun review is going to be a little off the normal structure. While the firearm we chose to use is important, the bigger picture, in this case, is what that means for the caliber. The previous sentence should actually read firearm(s) because it is two in tandem that really makes the case for what I am proposing as a radical departure from the norm in survival calibers.
From the picture, you can no doubt tell we got our hands on a CMMG AR-15 upper in 5.7x28mm. And like all the other CMMG products I have reviewed, it is absolutely fantastic. If you are a long time reader, I don’t have to tell you how highly I think of the CMMG 300 series firearms. I have already done so here, here, and here.
Just a few notes on the upper, before we get to the meat. I don’t mean to overlook the CMMG engineering here. It is amazing and has all the awesome features of the other CMMG products we have reviewed. Our test model was a 16.1 inch upper in Burnt Bronze cerakote, one of the 12 free cerakote options on any CMMG product. Instead of a gas impingement system, the Mk57 upper uses CMMG’s patented radial delayed blowback system. Just like on the 10mm pistol, it reduces recoil significantly, as well as runs like a sewing machine. Due to the lower pressure of a 5.7mm round, the bolt is skeletonized to reduce weight. And par for the course, it includes CMMG’s oversized Ambi charging handle. It’s fantastic, as has been every other CMMG product we have tested.
But the real story here, to me, is what this means for the cartridge. Especially when combined with a new handgun from Ruger last year, the Ruger 57. Either gun, by itself, pretty cool I will grant. But neither alone really enough to convince me to relook 5.7x28mm. But together? Now we are starting to look at a system that has the potential to turn survival caliber debate on its head.
Let’s start with the ballistics data, as that is particularly relevant to this discussion. 5.7x28mm from a pistol length barrel (The Ruger 57 has a 4.94-inch barrel) is fast, but not what we would call staggeringly fast. To deep dive this, we have to first take a step outside of our current gun discussion and address some terminal ballistics theory.
Now I say theory because terminal ballistics or wound ballistics is not a settled science. If it was, we would all be running a single caliber by now, military included. There has been a lot of testing, and a lot of debate, which does give us a starting point. We also have a lot of real-world experience shooting humans over the last 20 years, which matters too. But at the end of the day, we all still have to make our own decision. My take on the 5.7x28mm is fueled by my combat experience and what I can ascertain from scientific data, but a lot of it is still an assumption. While all that experience bit is real, the fact remains 5.7x28mm hasn’t seen much if any battlefield use, at least not by anyone I have met. So let’s call what I am going to say next an educated guess, very much open to new data.
In the great debate as it pertains to small, fast projectiles we come across two mechanisms of wounding. And you can go down some serious rabbit holes here, starting with Dr. Martin Fackler. The first principle is what is called a temporary wound cavity or secondary cavitation, depending on your preferred verbiage. The idea here is that the velocity of the projectile directly affects how much tissue stretches away from the point of impact. Tissue being largely water, it reacts to a high-velocity missile much like the ripples in a pond. You can see this in anything from a boxer hitting another boxer, to the flex of a gelatin block with a bullet. The principle is that the water/tissue flexes away from the impact, pushed not only by the force of impact but the air being dragged along by said projectile, especially in supersonic flight.
Where this theory gets dicey is in how much it matters. No one, ever Dr. Fackler, debates that it happens. But there are competing claims about how much it matters. Dr. Fackler theorized that secondary cavitation meant almost nothing, as the tissue usually just stretches and returns to form with minimal damage. Later theorists, however, dispute that. The second theory is that secondary cavitation does matter, provided the incoming projectiles speed is high enough. The idea here is that below a certain threshold of about 2400 fps, the tissue is undamaged. But above that threshold, the tissue stretches so much that it is actually torn and damaged, producing wounds all out of proportion to the size of the bullet.
The second theory of wounding comes from the principle of fragmentation. The idea here is that if you push a bullet fast enough it will come apart when it impacts tissue, as the g forces on the yaw of the bullet snap it apart into multiple smaller projectiles. Each with their own wound path. ( Yaw can be simplified like this. Bullet is flying nose forward toward tissue. The nose hits the tissue first, and therefore starts bleeding speed first. Like a car wreck, the rear of the bullet has not had time to slow down as the nose impacts. So as the bullet is penetrating tissue, it is also flipping over. In high quality ballistic gel you can often see this. Small wound path, a big one as the bullet is actually flipping over inside the tissue, and then the bullet exits base first.) Once again, for fragmentation, speed matters as does bullet design. It was a happy accident that M855 Green Tip, allegedly built with a steel core for barriers, does this at high velocity. The jacket rips off and goes one way, the steel penetrator another, and the base of the bullet yet another. The data varies on the velocity needed for most FMJ bullets to do this, but a rough estimate is between 2400 and 2550 fps.
Okay, so why the nerd out on ballistics? Because this is where it starts to matter. If we are talking about just a 5-inch pistol barrel, the 5.7x28mm round pushes a 40-grain projectile at around 1750 fps. Impressive, but below the thresholds mentioned for either wound theory as it relates to rifles. Does that mean 5.7 isn’t good enough in a handgun? Not exactly. There is one real-world example of the lethality of 5.7x28mm from a handgun, though I hate to give any credit to my enemy. When Nidal Hassan shot up Fort Hood, he was using an FNH 5.7. And while he was shooting unarmed Soldiers inside a building trying to do some BS paperwork, we also cannot ignore the statistics. 13 Soldiers were killed outright, and another 30 wounded. Considering 82% of those shot by handguns in the US survive normally, those figures matter.
Then we get to the rifle part of 5.7x28mm. While it was introduced in a pistol at the same time, 5.7 was always intended by FNH to fit in a submachine gun. Therefore, the caliber finds its ideal velocity in a 10.5-inch barrel such as the original P90. While our test model was a 16.1-inch barrel, CMMG does make both 10.5-inch uppers, and complete 5.7x28mm AR-style “pistols”. And in this platform, things start to change rapidly.
From a 10.5 barrel, 5.7 can achieve some absolutely staggering velocities. Round dependent, 2350 fps is fairly common, and some newer premium designs make it all the way to 2800. Which, if you know anything about short barrel 5.56, should immediately have you scratching your head. Why?
Because that very nearly matches what 5.56 will do. It is well documented that 5.56 starts losing velocity as barrel length is shortened from 20 inches. 223/5.56 was made for 20-inch barrels, and it starts falling fast as you chop that off. With the proliferation of AR pistols in 8 to 11.5-inch barrel lengths, with 10.5 being the most common, this should cause a serious rethink. It turns out, 5.7x28mm will very nearly match 5.56 if you are packing a shorty.
Which opens up a lot of trains of thought. First of all, since it was designed for the barrel length, 5.7×28 doesn’t create the dragon’s breath fireball of shorty 5.56. 5.7 has less recoil, as it is a lower pressure round. I can tell you from shooting it in the CMMG upper, it recoils like a rimfire. And oh Lord, the difference in weight of ammunition. Some of this is in the projectile, to be sure. 55 grain 5.56 vs 40 grain 5.7. But still, the 5.7×28 weighs roughly half as much per loaded round of 5.56×45. It isn’t unreasonable to say you could carry twice as many bangs per pound by switching to 5.7x28mm. And be able to share ammunition with your handgun, which is basically a logistics dream come true. And did I mention the normal mag size for 5.7x28mm in an AR holds 40 rounds? Plus 20 in your handgun, that is a lot of bang.
These new guns, in tandem, have the capacity to shape the future. Especially as we see the price of 5.7x28mm drop as its popularity goes up. (We of the Cult of the 10mm have been blessed with this over the last few years as well.) At least if the ammo market ever recovers. It has been impressive enough to make me re-evaluate the 5.7, and I hope it does you too. I have a feeling the little cartridge that could is just getting started.