Alternative AR Calibers

After discussing gas system options, Erin's other big AR question dealt with alternative calibers. Being as the AR is the Barbie of the gun world, there's a monstrously large list of chamberings for the platform, and swapping between them can be as simple as pushing two pins and changing uppers. At worst, you may also have to change a buffer as well. All in all, the changeover takes about 30 seconds.

With so many options available, it would be a Herculean task to list and break them all down. And while I may be mighty, Hercules I am not, so I'll stick to some of the more common and available calibers that the AR can run.

.22 Long Rifle
.22LR is cheap, very quiet, and has virtually no recoil in the AR-15 platform. It makes an excellent practice and teaching caliber, allowing the same trigger and control setup used in a centerfire rifle to be employed on a trainer.

6.5 Grendel
The Grendel was a fairly early entry into alternate AR calibers. It is designed as a precision long-range cartridge, and was an early standout in competition shooting. Unfortunately, it is less widely available than the other calibers on this list.

7.62 x 39
Delivering performance similar to the legendary 30-30 Winchester, the 7.62x39 adapts fairly well to the AR platform, turning a basic AR-15 into a very functional big game hunting rifle. It does however have some quirks, such as feeding from non-standard magazines, and difficulty with setting off the hard primers used in cheap imported ammunition.

.300 AAC Blackout
Designed by Advanced Armament Corporation, the Blackout is a 5.56 case necked up to .30 caliber, which means it feeds and functions using standard AR bolts and magazines. AAC is a suppressor manufacturer, so the Blackout comes in two loadings: the 125gr supersonic load performs very similarly to a 7.62x39 with none of the drawbacks, while the 220gr load runs at subsonic speeds and is specifically designed to run with a suppressor. If you have the means to acquire a suppressor, this is an excellent load at shorter ranges.

.458 SOCOM
If no other caliber on this list has enough power, the .458 SOCOM does. Using standard AR magazines at reduced capacity, this round was developed as a short range, very hard-hitting round for soldiers performing missions outside the performance characteristics of the 5.56. It also has both supersonic and subsonic loads, with the subsonic load delivering roughly twice the energy of the .300 Blackout. The supersonic loads also make a dandy short and medium range hunting round, suitable for almost all game in North America.

9mm Luger
Yes, the same 9mm Luger that runs in untold numbers of police and private citizen handguns also functions wonderfully in carbines. The added barrel length gives a substantial velocity increase, and the longer sight radius and stability of a shoulder stock contribute to greatly improved accuracy. In a carbine, it's even easier shooting and lighter recoiling than in a pistol. Unfortunately, it requires very different magazines and a block inserted into the magazine well to make the magazines fit and feed properly.


Selecting an alternate chambering for an AR allows you to tune your weapon for a specific purpose or to meet a particular need. The modular nature of the AR makes this a fairly straightforward proposition, and one very worth investigating.

Lokidude