The debate of whether the 5.56x45mm round is effective in the M16 platform has been around since they first came into service, replacing the M14 which sported the 7.62x51mm cartridge. The round and rifle have both proven themselves in the last 50+ years, and the debate (mainly) has been put to rest. In this post, I wanted to touch on the specifics of the 5.56mm round, and then discuss the round in Short Barrel Rifle applications, especially since they have been increasing in popularity as Personal Defense Weapons (PDWs). For the sake of simplicity, I will refer to the 5.56mm, but the same information applies to .223 rounds as well, although you should never shoot 5.56 ammunition in a rifle chambered for .223, while a weapon chambered in 5.56 can accommodate both 5.56 and .223.
5.56x45mm Wounding Mechanisms
The round is very effective in certain scenarios because of two effects not seen in most other rounds: tumbling and fragmentation.
At the proper velocity, the round tumbles and yaws when it enters the body, causing massive damage to the immediate and surrounding tissue. This creates a massive wound channel.
As the round tumbles, it also breaks apart, with the smaller pieces diverging out and further damaging surrounding tissue.
These two wounding mechanisms are how the round overcomes its shortcomings compared to other larger and heavier rounds. These mechanisms, however, are linked to velocity. Under a certain velocity, the round fails to tumble and fragment, in which case the main damage seen is a .224″ hole in the body.
Velocity and Barrel Length
Many people believe that shorter barrels are less accurate than longer barrels. This is false. In reality, shorter barrels result in lower velocities for the exiting bullet. Inside the casing, the primer detonation ignites the gun powder. As the powder ignites and heats, the gas inside rapidly expands, propelling the bullet down the barrel. Think of it as expansion of gas, not an explosion. The 5.56 was designed for the M16, which features a 20 inch barrel, and the bullet uses all of this length to get up to a velocity of 3,000 feet per second. So as the barrel length is decreased, the bullet exit velocity also decreases, since the gases don’t have enough time to get the bullet up to speed before the bullet is expelled. Here is chart showing barrel length vs velocity for the 63 grain M855 round (image courtesy of safedefensejournal.com):
Note the red line on the chart. Below this 2500 ft/sec, full metal jacket rounds will not reliably fragment. Basically, any AR with a barrel under approximately 9.5″ will never be able to expel a projectile over fragmentation velocity. A 10.5″ barrel will launch a projectile over this barrier, but air resistance/drag will quickly slow it down to under frag velocity in around 25-50 yards. Even going with an 11 or 12 inch barrel will still only get you to around 75 yards max. By the time you get a little longer, you effectively lose the advantage an SBR altogether.
At this point, if you have an SBR or AR pistol, you are probably getting depressed. You’ve put in a lot of money and time to get a cool tool that you would bet your life on. You may even be shooting sub MOA at 100 yards and now you think it wouldn’t be effective at that range. Well, a .224″ hole through your body is not something you easily walk away from. But, there is additional consideration: ammo selection.
Up to this point, all we have discussed is the military application of these rounds and rifle combination. The military has its hands tied with the Hague Convention (commonly misattributed to the Geneva Convention) which states that we will not use expanding or hollow-point ammunition in a war environment. Luckily, civilians are not bound by these rules and can run a variety of ammo.
The best choice for self defense ammo in SBRs is the expansive variety: Hollow Points (HPs), Soft Points (SPs), and copper expanding bullets are all good options for your load out. I would advise to stay away from Open Tip Match (OTM) rounds, as most were primarily designed for superb accuracy and a super flat trajectory, but not for penetration or expansion. A good resource for ammo specifics can be found here: Self Defense Ammo by AR15.com
And a vast amount of info on wound ballistics and ammo choices: Wound Ballistics and Accuracy of Ammo
BarnesTSX 70 gr Copper Projectile
My SHTF weapon is a 10.5 AR-15 SBR in 5.56. I do have a 300 Blackout in the same length that I would prefer, but finding ammo if I was forced from my home would be near impossible. And even if I ran out of my expanding ammo for the 5.56 and had to use milsurp ball ammo, it would still beat throwing rocks. You may ask why I wouldn’t choose my 16″ AR-15 for my get-out-of-Dodge weapon. For starters, if I were to need to use my weapon in a defensive situation, it would be close quarters, as anything farther would warrant Escape and Evasion maneuvers, in which an SBR would be more handy. Also, after doing a few bug out drills with that rifle which included 10+ miles of humping my ruck and weapons, I decided having something light and maneuverable was more important than a few hundred FPS. Ounces equal pounds, and pounds equal pain, as they say.
After you have chosen some ammo, make sure your rifle eats that ammo like candy. It wouldn’t hurt to pick out a few different types of ammo (FMJ, HP, SP) and stagger them in your magazines. The FMJ would suffice if you need penetration through an object first (glass, door, etc.) then the expansive type for direct contact. No matter what you choose to keep for your SBR, range time and training are always just as, if not more, important. In the words of Wyatt Earp, “Fast is fine, but accuracy is final”.