For most guns, whether they're bolt action rifles or semi-auto pistols, "what caliber is it chambered in?" is a pretty simple question. For the AR 15, one of the most modular and popular platforms ever produced, the answer is a little trickier.
While the original AR 15 rifle was specifically designed by Eugene Stoner to be used with the then-experimental 5.56x45mm cartridge, which was later adopted as the NATO standard, the AR 15's popularity in both the military and civilian world has led to manufacturers chambering the rifle in dozens of different calibers since then.
Though the full range of AR platform cartridges is beyond the scope of a single article, we've compiled a list of some of the most common and most popular options, along with the basic pros and cons of these different cartridges. Keep in mind that we're mostly focusing on factory ammunition - there's so much variance in bullet weight and powder charge that accounting for all of the handloading options would be impossible.
5.56 NATO / .223 Remington
The granddaddy of AR 15 rounds, the 5.56x45 NATO / .223 Remington is an excellent all-around choice, capable of being used in home defense, tactical, and hunting roles with ease. Designed to overcome the heaviness and control issues of the previous 7.62x51mm cartridge used by the M-14 rifle, the high-velocity 5.56 NATO cartridge was well-suited to the intermediate range engagements that the US Military found itself in after World War II.
If you're in search of a versatile cartridge that can tackle most tasks from deer hunting to HD use, feeds reliably, and has more aftermarket support than any other platform, there's absolutely nothing wrong with taking the classic 5.56 NATO route - especially since it allows for some easy conversions later down the line if you decide to try something else. Ammunition is readily available and reasonably priced, and you can find an AR 15 rifle chambered in 5.56 basically anywhere that guns are sold.
It's the gold standard for a reason, but if you're looking for more specialized cartridges, then read on.
If you're looking for a way to turn your AR 15 into an incredibly affordable plinker or varmint gun, then look no further than a .22 LR conversion. While there are dedicated .22 uppers available, it can also be done even cheaper by simply swapping out the BCG and buffer system - in fact, CMMG offers the .22 LR Bravo Conversion kit that includes everything you need, including a magazine.
An AR 15 chambered in .22 LR is a great way to train new shooters or simply hone your own skills on the cheap - at pennies per bullet, it's ideal for doing muscle memory drills without breaking the bank, and it's easier to get set up than most other AR 15 pistol calibers. And of course, it's also an excellent varmint cartridge.
Arguably the most popular AR 15 caliber conversion involves turning into a 9mm. It's not hard to see why: 9mm is the most popular centerfire pistol caliber, and is typically much more affordable than 5.56. And for defensive use, there's also something to be said for having your rifle share ammo with your pistol.
Converting your AR 15 rifle into a 9mm is a bit more of an involved process that the .22 conversion, however. You'll need to either swap barrels or buy a dedicated 9mm upper, a 9mm BCG, a heavy buffer and heavy buffer spring (a 9mm AR doesn't use the typical gas impingement system, and uses blowback to cycle the bolt instead), and a magazine conversion kit that allows you to feed 9mm mags into a standard AR15 lower.
Alternatively, you can go with a dedicated 9mm AR lower instead.
Pistol calibers may seem like an odd choice for an AR 15, but they can be a great way to practice or simply enjoy your guns when ammo prices become restrictive.
Developed from the 6.8 SPC parent cartridge, .224 Valkyrie is a relatively new kid on the block when it comes to AR 15 rifle calibers. However, .224 quickly proved its mettle in the form of a longer, heavier bullet that could buck the wind and still boasts an extremely flat trajectory thanks to its exceptional ballistic coefficient.
.224 was built for long distance use, and unsurprisingly, that's where it excels. Bullet drop is much lower, wind drift is less noticeable, and shots are overall more predictable, even when pushing out past 1000 yards. If you want an AR 15 that is capable of going toe-to-toe with a target shooting bolt action rifle, .224 Valkyrie is a good place to start.
.300 BLK was developed by Advanced Armament Corporation and Remington in conjunction with US Special Operations Command as a way to provide a rifle round that had better terminal ballistics out of a shorter barrel, could be used suppressed or unsuppressed with both supersonic and subsonic loads, and would little modification for use in the military's standard issue M4 platform.
The cartridge delivers on all of those promises, and its adoption by US Special Operations has sparked a growing popularity in the civilian market as well. Because .300 BLK plays nice with the standard AR 15 BCG, buffer, and magazines, all you need for this conversion is a new barrel or a new upper, both of which can be found at a very reasonable cost.
I would recommend picking up some dedicated .300 BLK magazines, even though you can use the standard 5.56 just fine. You'll get slightly smoother feeding, but most of all you'll avoid the risk of accidentally slapping a mag full of .300 BLK into an AR 15 with a 5.56 upper, which will leave you looking for a new rifle at best and a hospital at worst.
Developed by Alexander Arms and first unveiled at the Blackwater Training Facility in 2003, the 6.5 Grendel round was conceived as a way to take advantage of the excellent ballistics of 6.5mm bullets by necking down the 7.62x29 round.
In tests against the 7.62 NATO cartridge, 6.5 Grendel outperformed it in terms of both terminal and exterior ballistics by shooting flatter, penetrating further and staying supersonic over a longer distance - all with lower felt recoil on the shooter's end.
It's a great round for hunting and competition shooting - in fact, it closely resembles the .264 USA round in use by the US Army Marksmanship unit, and is capable of excellent accuracy at long range.
There are a few downsides, though. Ammunition is expensive, and if you want a barrel length under 20 inches or a magazine capacity higher than 15, expect to do a bit of searching and spend more money than you would with most AR caliber conversions.
6.8 SPC is a bit of an oddball round, functioning as a sort of jack-of-all-trades that never quite gained a major foothold in the market. Originally developed for the US military, the round was meant to provide superior ballistics and increased stopping power at closer range.
While the 5.56 round is certainly fine in CQB situations, the cartridge was designed to be fired from a 20-inch rifle barrel, and when shorter carbine and SBR platforms started seeing use in the urban environments of Iraq and Afghanistan, the ballistics of the 5.56 round suffered.
In that regard, 6.8 SPC is actually successful. It retains significantly more energy than 5.56 when fired from a short barreled rifle, and packs a bigger wallop when it hits. And while it might not outperform rounds like .300 BLK that were specifically designed with SBRs in mind, it does outperform that round at longer ranges.
Though this cartridge never ended up being widely adopted by the military, the civilian market has given it a new lease on life. Converting an AR 15 to 6.8 SPC requires a new upper, bolt, and magazines, and in exchange you'll find yourself with a rifle that makes for a great hunting option in states that prohibit 5.56 / .223. Whether you're deer hunting or bagging wild boar, even factory loads are plenty powerful enough for a clean kill.
It may seem like sacrilege to chamber an AR in the cartridge made famous by the AK47, a firearm which is opposite in almost every way to the AR 15 from a design perspective, but hear me out.
In addition to its excellent performance inside of 300 yards, a AR 15 rifle chambered in 7.62 is a great alternative if you want to go hunting in a state with strict minimum caliber requirements. While it's certainly not the best choice if you're chasing accuracy at longer ranges, it will deliver a serious punch at short and intermediate ranges, and is more than capable of taking down deer-sized game. Ammo is affordable, and honestly, it's fun to shoot, even if the recoil is much more noticeable.
On the other hand, converting an AR to 7.62 requires a pretty substantial investment. Some options, like the popular PSA KS-47 upper, require the use of a proprietary AR lower, and you'll need a new BCG as well. You may even need to swap out hammer springs for a higher-powered version, since 7.62 primers are quite hard and may cause failures to fire with a standard AR trigger setup.
Now we've reached the big boy cartridges. A necked-down version of the .50 AE cartridge made famous (or infamous) by the Desert Eagle, .458 SOCOM packs a lot of punch into a relatively little package. If you want to hunt with subsonic rounds, you'd be hard-pressed to beat this cartridge - inside of 75 yards they're great deer hunting rounds, and with supersonic ammo the ballistic profile closely resembles the venerable .45-70.
As the name implies, this round was developed after the Battle of Mogadishu, when members of US Special Operations Command lamented the poor performance of the 5.56 round in an extremely close range urban environment. .458 SOCOM offered a solution in the form of a big bullet that functioned very well out of the AR platform regardless of barrel length.
Like other big-bore AR 15 calibers, ammunition is on the pricey side, though the bullets themselves aren't too costly if you have an established reloading setup.
Another creation by Bill Alexander, who also developed 6.5 Grendel, .50 Beowulf is a monster of a cartridge and a great hunting option. In fact, even with factory ammunition it is capable of taking any big game you'll find in North America, and much of the game in Africa, too! But like other big-bore AR cartridges, .50 Beowulf really shines when it comes to hand loading your own ammunition, where the robust powder capacity allows for some screaming-hot loads.
The first thing you'll notice about the .50 Beowulf cartridge is the odd case shape. It's a thick, straight wall cartridge with a severely rebated rim, and unlike other calibers common to the AR 15 that come to a fine tip,.50 bullets look like the blunt sledgehammers they are.
Ballistics are pretty much what you'd expect from any kind of cartridges with these massive bullet weights - lower velocity and fast dropoff in exchange for enormous stopping power and very little deflection.
Those qualities made it a popular choice for military use at vehicle checkpoints, where bullets that could punch through auto panels and windshields without deflecting could be the difference between life and death.
Is it the most practical gun in the world? Not unless you make a habit of shooting bull moose and grizzly bears. The bullets are too expensive for casual plinking and completely impractical for personal protection. But is it a lot of fun to shoot? Absolutely. Recoil isn't as bad as you might expect, and much more manageable than other .50 guns, but it's still a serious thumper of a cartridge, and it's sure to put a smile on your face. Sometimes, that's the point.
And while the ammo itself will set you back considerably (though substantially less so if you don't mind reloading), a .50 Beowulf upper can be had at a pretty reasonable price point, especially compared to the cost of other .50 rifles.
The AR 15 is a time-tested, versatile, and reliable gun, and the ease with which it can be converted to a wide variety other calibers is a testament to that.
Whether you're hunting big game in Alaska or doing some competitive long range shooting, whether you need a varmint cartridge or subsonic rounds for CQB use, the AR 15 platform has something to offer for everyone.
Which one is the best option? That depends on your intended purpose. For example, someone who just wants to have an affordable, fun day of plinking might find a .22 conversion to be the perfect answer, while someone looking for a gun to bag wild pigs or shoot sub-MOA groupings at targets 500 yards away would need completely different cartridges.
We hope this article has helped you sort out the pros and cons of these cartridges. While there are even more options available than the ones we've listed, we tried to cover all of the most common and most popular picks among the different calibers. If we missed your favorite, sound off in the comments and let us know!
And if you're looking for a good article on some of the builds you can put together without breaking the bank, check out our list of the 10 best AR-15 rifles under $1000!