Firearms are really defined by their cartridge, so if you want a gun that will do most things, focus on a round that serves well.  The original AR round, the 5.56/.223 was designed from the ground up as essentially a jungle warfare cartridge.  Its use has expanded far beyond that scope today, and it is a fantastic cartridge, but there are some things it doesn’t really do well.

The AR pattern Modern Sporting Rifle is without a doubt the most popular rifle in the US today, as we have discussed in the past it is an extremely versatile design with many positive attributes.  But it is not without its’ limitations.  Firearms aficionados love a challenge, so the question is, how to improve on this already great system?

Standard AR15 pattern rifles have two main limitations: overall cartridge length and standard case head diameters of  .378 inch. If the cartridge is too long then it will not fit in the magazine and it will not safely cycle or chamber in the receiver; likewise, any deviation in case head size requires modifying the upper receiver and bolt.  This puts a practical limit on the cartridge selection. In order to get into truly high power cartridges one has to step up to the AR15’s larger brethren which are modeled after the AR10 pattern, which is an expensive proposition. For this article I am also not going to get into the AR style rimfire, or pistol caliber centerfire rifles.  They exist and are a lot of fun, but that would overly complicate the article. Be aware there are simple conversion kits that allow a shoooter to quickly adapt their standard AR to shoot 22LR ammo. That is a very cheap and fun way to be able to spend a day at the range.  This article will instead focus on alternate centerfire rifle calibers:


The Standard:

5.56 Nato/.223 Remington 

Before anyone gets pedantic, I know they are not the same, but for practical purposes they are.  Most MSR’s (Modern Sporting Rifles) are marked as 5.56 which means they can safety shoot .223 Remington ammo.  This is the original caliber for the platform. It is generally (other than .22 LR) the cheapest to buy and the most plentiful.  It is great for long range sessions and lots of fun to shoot. Recoil is negligible, and accuracy is great. Obviously they are good for home defense with proper bullet selection, and their use on varmints and coyotes is superb.   Also unlike the rimfire cartridge, the 5.56 can be reloaded, or the brass can be saved to sell to someone who reloads, which can recoup some of your costs.  The real focus of the article are the cartridges listed below, but remember, the standard chambering is really an excellent round within its limitations.  As always your friendly local neighborhood ballistic arts emporium has several good deals on new and used options in stock

Ultra High Speed Low Drag:





.22 Nosler –

A new-ish and blisteringly fast round that has the speed of a .22-250 out of a package that will fit in an AR15.  The case is the same length as the standard 5.56/.223 round but is a little girthier, making more room for powder, and velocity.  This velocity will create slightly flatter shooting than the 5.56/.223 and should make the varminters and long range shooters happy.  The cartridge is new, and proprietary, so unless you are serious and want to commit to the expense of the speciality ammo, stick with the standard stuff.  Ballistics really favor the Nosler past 400 yards though. Converting a standard AR to .22 Nosler involves using 6.8 SPC magazines, and picking one of these three choices 1) a different barrel ($$); 2) a complete .22 Nosler upper ($$$); or 3) professional gunsmithery to your existing barrel ($$),  After that you will need to also retune your gas system ($).

Requires: Different Barrel, 6.8 SPC Mags, and Retuned Gas System

.224 Valkyrie –

Based directly on the 6.8 SPC cartridge (See below) instead of being totally proprietary, (looking at you Nosler) the Valkyrie requires a special bolt because of its’ larger than standard bolt face. The Valkyrie is another .224 caliber screamer designed for seriously heavy-for-caliber 90 grain bullets making it a viable 1000 yard target caliber.  Of all the .22 caliber bullets the Valkyrie is the only one I would consider using on deer, and only then if it was legal AND I was sure of my abilities and had a good shot. But remember 90gr Sierra Matchking bullets are what has killed a lot of deer out of a .243, something to think about. Between the Nosler offering and the Valkyrie I think I would have to give my nod to the Valkyrie. Again, to convert an AR to shoot Valkyrie, you would need a new bolt, a new barrel, and retuning your gas system.  You could buy a complete upper, or even a complete rifle. You are going to have to use 6.8 spc magazines.

Requires: Different Bolt, Barrel, 6.8 SPC Mags, and Retuned Gas System

The Contender

Ahhh The .300 Blackout – what happens when you take a .223 case and blow the neck out to .30 caliber (7.62mm)?  Well, with fast burning powder, and a 150 gr bullet you get a rifle that mimics the ballistics of an the AK round and is similar-ish to a .30-30.  A good little deer or hog cartridge for fairly short shots, say inside 110 yards. It will ring steel out to 200 with very little effort. Load it for subsonic ammo and 220 grain bullets and velocity drops to less than 1000 fps.  You have converted your tactical rifle into a pistol caliber carbine shooting the ballistic equivalent of a .45 ACP. Attach a suppressor and the loudest sound you will hear out of it is the slamming of the bolt, and the tearing of paper targets.  The .300 blk has more downrange power than a .223 or any .22 will ever have, at the expense of range. The best part about it, the only thing that differentiates a .300 blk rifle from a 5.56 rifle is the barrel. Same mags, same bolt, same gas system (unless you want to run suppressed, then it works better with a shorter gas system).  This may be my favorite caliber for ARs. It is great for defensive use (especially subsonic), good for hunting hogs and deer, and the price of the ammo and the rifles themselves is getting near to what 5.56 costs. Since it is made directly from 5.56 brass and was designed specifically to work in standard AR-15 magazines, it works flawlessly with standard AR-15 bolts and magazines. There are no issues with feeding, no issues with magazine capacity, and spare parts are abundant since the only real difference is the barrel.  I know several people who have bought 300 blackout rifles from Liberty Tree Guns, all are happy with them.

Requires: A different barrel

The Comrade

  7.62 x 39 (AK) –

The near ballistic twin of the .300 blk, it was the USSR’s go to service rifle round from the end of WWII until the early 1980’s and is currently used by most tier three militaries and paramilitary groups in the world.  The caliber is not ideally suited to the AR platform, mostly due to its case taper. This taper requires curved magazines and generally leads to poor feeding in AR style rifles. Some manufacturers have made it work, using some specialized parts.  My best advice to someone is that if they want to shoot this round: buy an AK style rifle, (you can get them you know where) alternately shoot .300 BLK in an AR. Obviously the AK round is great defensively, and in a good barreled rifle will shoot similarly to a .30-30 for hunting use.  Ammo is still readily available, but not as cheap as it once was.

Requires: Bolt, Barrel, Mags, Special Lower, possibly more.

The Wonder Twins

6.5 Grendel –

The 6.5mm Grendel’s design goal was to create an effective 200–400 yard AR-15 magazine-length cartridge for the AR-15 that surpassed the performance of the native 5.56mm NATO/.223 Remington cartridge in military situations. For hunting the 6.5 Grendel is a very capable 300 yard deer round, but it is not in the same class with the .308, .270, etc. Firing factory loaded ammunition loaded with bullets ranging from 90 to 129 grains, its muzzle velocity ranges from 2,500 ft/s to 2,900 ft/s. The case head diameter of the Grendel is larger than the 5.56×45mm NATO, thereby necessitating the use of a non-standard AR-15 bolt. The increased case diameter results in a small reduction in the capacity of standard size AR magazines. A Grendel magazine with the same dimensions as a standard 30-round 5.56 magazine will hold 26 rounds of ammunition. 6.5mm bullets have a high sectional density, meaning they are very aerodynamic and thus lose velocity very slowly and are affected by wind less than other cartridges.The Grendel sits in the middle performance wise between the .257 Roberts and .243 Winchester which makes it a very good all around cartridge. To get the best performance from this type of cartridge from an AR it is advised to outfit yourself with a 20” barrel .

Requires: Different Barrel, Bolt,.

6.8 Remington SPC 

Based upon the .30 Remington cartridge, it is midway between the 5.56×45mm NATO and 7.62×39mm in bore diameter and muzzle energy. It uses the same diameter bullet as the venerable .270 Winchester hunting cartridge; though with less power (think of it as a .270 short).  This makes handloading bullet selection generally excellent. Adapting an AR-style rifle to the new cartridge requires the replacement of the barrel, bolt, and magazine.The 6.8 SPC delivers 44% more energy than the 5.56 NATO at 100–300 yards. The 6.8 mm SPC is not the ballistic equal of the .308 but it has less recoil, and is not as aerodynamically efficient as a comparable weight projectile of 6.5mm like the Grendel. That being said it is really a matter of personal preference between the 6.8 SPC and 6.5 Grendel.  To get the best performance from this type of cartridge from an AR it is advised to outfit yourself with a 20” barrel

Requires: Different Barrel, Bolt, Magazines



.450 Bushmaster 

With a bullet diameter of .452″. It uses a rebated rim cartridge case based on the .284 Win. case.  You can use .45 handgun bullets if you are careful in your selection and loading, and there are some good loads on the market. If you want readily-available factory ammo, .450 Bushmaster might be the best bet. The 450 bushmaster requires some proprietary parts such as magazines and bolts.  But it is a good choice if you are looking for serious power out of an AR15

Requires: Magazine, Barrel, Upper, and Bolt

.458 SOCOM –

Essentially delivering .45-70 performance out of a modified AR15 platform.  This thing is a beast. Created from requests for a heavy hitting cartridge from the Rangers working in Mogadishu it was developed in Broken Arrow Oklahoma in 2001.  The .458 is based on the .50 AE handgun cartridge fitted to a .458 bullet and operating out of a special AR15 upper receiver. A standard lower and standard AR magazines work, however, capacity is obviously much lower; a standard 30 round AR mag holds just 10(!) .458 SOCOM slugs. One big advantage of .458 diameter is there is a wide selection of bullets on the market, with bullet weights ranging from 140 to 500 grains. It’s more powerful than the .450 and more versatile than both the .450 and the .50 Beowulf. You can duplicate .45-70 loads or create subsonic 500-600 gr thumpers.  Of the heavy hitting AR15 calibers, this one seems to be the best mix of convenience, versatility, and power.

Requires: Upper, Barrel, and Bolt

.50 Beouwulf –

For those who have to have the biggest with the mostest, this is the biggest caliber currently offered in an AR15 size platform. Designed to be a vehicle stopper.  It is best described as a low-velocity, heavy caliber, making its ballistics roughly equivalent to those of early .45-70 Government rounds.  It is an entirely proprietary cartridge from Alexander Arms, meaning they are the source for parts and ammunition. Adaptability is limited due to its use of the 7.62×39mm (AK style) bolt face

Requires: Upper, Barrel, Bolt, Mags

There you have it, the currently common-ish alternative chamberings to feed your AR15 addiction.  The real standouts in my opinion are the .300 blk and the 6.5 Grendel, but your mileage may vary, and my opinion is only that: an opinion.

Cartridge Bullet weight MV Energy 0 ft Energy@100 yds Energy @ 200 yds Energy @300 yds Energy@ 400 yds
5.56/.223 (5.56 x 45) 68 2960 1323 1098 906 741 600
.300 blk (7.62 x 35) 125 2175 1313 1036 810 609
7.62 x 39 (AK Round) 123 2300 1445 1070 778
6.5 Grendel (.264) (6.5 x 39) 123 2580 1818 1586 1379 1193 1028
6.8 SPC (.277) (6.8 x 43) 120 2460 1612 1349 1120 923 756

As always, Stay Safe, Stay Free