The AR-15 in 5.56 is the perfect place to start if you are a new AR owner or a new AR builder. 5.56 is kinda like the girl next door. It’s sweet, always there for you, and reliable. However, it’s also kind of vanilla. The good news about the AR platform is that it seems to be almost endlessly adaptable. You can go from teeny tiny 22 LR, all the way to the 50 BMG. (Seriously, this is an option) What I’ve gone ahead and done is put together 8 different calibers the AR can be chambered in using a standard multi caliber lower receiver to give you a taste of the AR’s adaptability.
The pistol caliber carbine has fully swept through the gun world, and the AR platform was not spared. There has always been 9mm ARs, but they were more or less specialty guns. Now everyone and their mom is making ARs in 9mm. A lot of interesting and dedicated lowers exist for a variety of different magazine types. Most commonly these lowers are designed to make use of Glock and Colt SMG magazines. There are a few more options out there from smaller companies too. You can also use a dedicated magazine block to convert a standard 5.56 lower to a particular magazine type.
The cool thing about 9mm ARs is you don’t need a gas system since they are blowback models. This makes it easier to build and to maintain them. Blowback isn’t refined, but it is reliable. Pistol caliber ARs are also much easier to suppress with easy to find subsonic ammo, that doesn't require you to tune the gun. Since it's a pistol caliber, you can also shoot at most indoor ranges. You'll find a lot of indoor ranges aren't set up for traditional rifle rounds.
On top of all this, there is noticeably less recoil, muzzle rise and concussion than regular ARs. This makes them easier for smaller or less experienced shooters to handle. 9mm ARs are great little guns and a great addition to any AR collection.
Of course, the plinker’s cartridge has their own AR. Building a 22 LR rifle is pretty easy and there are multiple methods to go about it. If you have just a fun gun and want to shoot your standard 223/5.56 AR on the cheap a simple bolt conversion and magazine is all that’s needed. These bolt conversions are odd looking because they include an entire self-contained bolt, bolt carrier and buffer group to make 22 LR function in a standard 5.56. These bolts just replace your standard BCG, and you utilize a special magazine.
CMMG ($229) Makes one of the best kits because it's nickel boron and 22 LR is a dirty little round, so it’s easy to clean and runs great when it’s dirty. These kits are pretty cheap and well worth it in ammo savings.
Alternatively, you can build an AR from the ground up in 22 LR. Like the 9mm version you don't need a gas system, and with 22 LR you'll use the same bolt and magazine you'd use for a conversion. The benefits on this is a much more accurate and consistent rifle. You can also forgo the buffer assembly. If you’ve ever seen the film Rogue One, Captain Cassian Andor carries a buffer tube free, short barreled AR pistol. If you ever want to replicate the build a 22 LR dedicated build is the perfect option.
300 Blackout is the bell of the ball of new AR-15 calibers. Introduced only a few years ago the 300 AAC Blackout round has been pretty successful. There’s ARs, single shots, and bolt actions all built around the 300 Blackout cartridge. A measure of its success is certainly due to the ingenuity of its design. The 300 Blackout can work with most standard AR components. This includes the mags, bolt carrier groups, buffers, lowers, uppers, and basically everything besides the barrel and gas system.
This makes it pretty cheap to use existing parts and systems to build an AR in 300 Blackout. 300 Blackout doesn’t exist to replace the 5.56, but to compliment it. 300 Blackout is designed to function in much shorter barrels and reaches its max potential in barrels as short as 9 inches. The 300 Blackout also has a wide variety of ammo available, including both supersonic and subsonic loads. This means the rifle is pretty easy to suppress. The 300 Blackout is an excellent overall caliber for someone looking for something a little different without a significant investment. It's ideal for home defense, hunting, and having fun.
300 Blackout is a fun cartridge and one built to last. The biggest downside is the expensive ammunition.
7.62 X 39mm
The old Commie Classic 7.62 x 39 is a cheap, easy shooting and potent round for medium range shooting. The 7.62 x 39mm is a capable little round at the ranges it's intended for. This little round has a bit more energy and oomph than the standard 5.56. It's also the cheapest 30 cal round I know of. The biggest downside is, of course, the limited range, it's actually designed to be used at 300 meters or less. You can use a standard AR multi cal lower with this caliber but need specialized magazines. This is due to the taper caused by the 7.62 x 39 mm round, so the magazines are bent in a rather odd way.
It’s also usually good to have a dedicated 7.62 x 39 firing pin that's strong and slightly longer than your average firing pin. Those hard primers can be a challenge for normal firing pins, so an extended pin works best. You may also want to go with a dedicated 7.62×39 LPK with a more powerful hammer spring for the same reason. If you are willing to solve the odd little issues with the 7.62 x 39 AR you can combine east and west to make a very interesting weapon.
Okay, so there is only 1 410 AR-15 that I know of. It's made by ATI and I had a cool opportunity to give it a test at Shot Show’s Range Day. This little AR-15 is based on a standard AR-15 lower and is a blowback weapon. This means you don’t have to deal with a gas system. Is there any real usefulness to an AR-15 in 410? No probably not compared to a standard shotgun, but the biggest question is, is it fun? Oh hell yeah it is. I tend to get giddy over shotguns in general, but if you make an AR-15 and chamber it in 410, I get downright excited.
If you don't have a shotgun this could be an interesting upper option in the future, however, as of now, ATI is only selling complete guns. They are built on the ATI hybrid poly lowers, and the whole gun is pretty affordable. Since 410's resurgence, this makes the gun just another option in a long line of AR options. If this gun is successful it’s likely we’ll see more than one company begin producing AR-15s in 410.
The 458 SOCOM is a big bore AR-15 round that is designed to function with a standard AR-15 lower. The round will even function in standard AR-15 magazines, albeit at a much lower capacity. A 30 round 5.56 magazine will only hold about 10 rounds of 458 Socom. The 458 SOCOM is a powerful round designed to take down targets fast, be it two-legged predators, or four legged beasts.
There isn’t much you have to do to differentiate between a 5.56 lower and a 458 SOCOM rifle. The 458 SOCOM is going to need its own upper receiver but is at no risk of damaging a standard 5.56 lower. The 458 Socom is an excellent hunting round, and perfect for taking hogs. The ammo is certainly not cheap, but it's definitely one of the most affordable big bore rounds out there, and one of the more common. Recoil is significantly more stout than a 5.56 round, but it’s not uncontrollable. A solid muzzle device can help a lot with reducing recoil, and a muzzle break is the way to go.
The 6.5 Grendel was designed in an interesting time in the early 2000s. It came to be at a time when people were really looking for a more powerful intermediate round for AR style rifles. The 6.5 Grendel fits perfectly in a standard multi cal lower but uses unique magazines. The 6.5 Grendel as a cartridge is actually well suited for up to 800 yards. A good 24-inch barrel will maximize the ballistic potential of the 6.5 Grendel round, but shorter barrels are still suitable. The 6.5 Grendel was and advertised for use with police and military use, but didn’t see much success in that market.
The 6.5 Grendel has become a popular hunting round for AR-15s. It’s perfect for medium game like deer and hogs, and it has a pretty low recoil for a hunting rifle. The 6.5 Grendel is actually more efficient than the 308 at longer ranges and delivers more terminal performance. AR-15's built on the Grendel platform don't necessarily require anything special, and a standard complete lower will function fine. You'll need a 6.5 barrel, gas system, BCG, and magazines of course.
The 6.8 special was the other early 2000s attempt to add a little more oomph to the AR-15. The 6.8 Special was developed by Remington and the U.S. Army Marksmanship Unit. So right away the caliber took a little advantage over the 6.5 Grendel in terms of mass producibility. One of the biggest goals was to produce more efficient ballistics from the shorter M4 barrel. An AR-15 built around the 6.8 Special is going to give you significantly more powerful round. The 6.8 Spc is also 40% more effective than the 5.56 when fired from a 16-inch barrel. The 5.56 was designed around a 20-inch barrel, but the 6.8 was designed for multiple barrel lengths.
Of course, you can go 20 or 24 inches to maximize even more ballistic power for the 6.8 Spc. The 6.8 Spc is also pretty consistent and accurate out to 800 yards. The 6.8 Spc does need its own proprietary magazine, BCG, barrel, and gas system. You’ll be able to use the standard 5.56 lower, buffer assembly, and of course standard lower parts.
How Many More?
This isn’t even a comprehensive list of all the calibers the AR-15 can be chambered in. It’s insane how adaptable a single rifle can be. With the AR market being so large you can find any combination and any caliber within reason. This list is just some of the most popular options that don’t require intensive special parts and can be built on a standard multi cal lower.
- Quarter Circle 10 AR-15 Glock Small Frame Lower Receiver ($279)
- CMMG 22LR AR Conversion Kit Bravo ($229)
- Rainier Arms RUC .22 LR Upper ($684)
- Rainier Arms Ultramatch PDW Pistol-8.5″ 300 Blackout TC Enhanced with Cheek Rest ($1999)
- Gemtech GMT-300 Silencer 300 Blackout ($949)
- Faxon Firearms AR-15 ARAK-21 Upper Receiver 12.5″ Combo 5.56 and 7.62×39 ($1519)
- Hornady Superformance GMX Ammunition 6.5 Creedmoor 120 Grain GMX ($35)
* Items may have been out of stock or discontinued at the time of this writing.