For most guns, whether they're bolt action ar pattern 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 assault 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 as a military rifle and civilian worlds has led to manufacturers chambering the semi automatic rifle in dozens of different calibers since then.
Though the full range of AR-15 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.
WHAT CALIBER IS AN AR 15
The answer to this question is difficult to answer because manufacturers have chambered the armalite rifle in several different calibers over time. However, the average AR 15 is usually chambered for a 5.56 NATO or a .223 Remington. It has also been produced in a .22 LR, a 9mm, and .224 Valkyrie.
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 armalite rifle share ammo with your pistol and pistol grip.
You can also use this in home defense situations.
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 match 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 ar pattern rifles.
The .350 Legend is primarily used as a hunting round for deer, particularly in states that only allow straight-walled cartridges for hunting those deer. It may seem odd, but there are a few states in the U.S. that don’t allow you to hunt with tapered cartridges, which rules out most rifle ammunition.
Even though the Legend has decent ballistics, it probably would not exist at all if it were not for those state laws. It does retain more energy as it travels than the .300 Blackout, but its effective range is essentially the same, and one is not inherently better than the other.
The ammo is harder to find, though it is getting easier, and it can be more expensive, but none of that really matters because if you’re in a state that requires it you can really only choose between the Legend and the Bushmaster. If you’re not in a state that requires it, chances are the round doesn’t interest you much in the first place.
First off, the Ham’r is expensive. Depending on the brand and load that you get, it could be the most expensive AR-15 ammo size on this list. That said, the round allows you to get the same ballistics as with a .30-30 Winchester out of an AR-15.
What does that mean? Well, it means that the name “hammer” is apt. If you’re shooting at less than 300 yards, this round is getting to hit like (you guessed it) a hammer. Ammo availability is scarce, since the only company who makes it is Wilson, which is probably one of the reasons it’s so expensive.
Wilson is keeping the round proprietary, so no one else can manufacture the ammo or hardware compatible with it. It’s unfortunate, because the round does well enough that it would probably see much wider adoption if they were willing to open things up a bit.
So why get the Ham’r? Well, if you’re shooting at under 300 yards, it’s probably the single best performing round in the AR-15 class. You get a lot more stopping power than you would ever think possible out of an AR-15. Wilson has high standards for their factory ammunition, and you’ll get great accuracy and precision out of every load they offer.
As you can probably tell from the photo, the .204 Ruger is designed specifically for varmint hunting. The projectile is quite small, ranging from 32 grains to 45 grains, which is one of the reasons it’s good for varmints. The recoil on these loads is minimal, and the muzzle velocity is incredibly speedy; sometimes north of 4,000 fps.
For a lot of shooters the .204 Ruger strikes the right balance between the .223 Remington and the .22 LR for hunting prairie dogs and other varmints. The .22LR starts to drop real quick as you get closer to 200 yards, and the .223 can feel like overkill (literally). The .204 offers great speed and ballistics compared to the .22LR, and is lighter and easier to take follow-up shots with than the .223.
That said, it fits the definition of a “niche” round, and it can be hard to find a pre-built AR chambered in .204 Ruger. It’s not hard to build one, so if that’s your thing then you won’t have any problem with it, but it’s not something you can waltz into a Sportsman’s and pick up on a whim.
ARC stands for Advanced Rifle Cartridge, which gives you a little insight into Hornady’s purpose in developing it. It was originally developed in conjunction with the U.S. military as part of their multipurpose battle rifle program. Like many other rounds, the ARC was built to get as many of the advantages of larger rounds while keeping the advantages of smaller rounds.
Unlike many of those rounds, the ARC has mostly succeeded. Performance-wise, it’s a lot like the 6.5mm Grendel, but with a wider variety of projectiles available for it, which means a rifle chambered in ARC will be more versatile than one chambered in Grendel.
It has significantly less recoil than a .308, while offering a package that is 30-35% lighter, but it also has significantly better ballistics than the 5.56, and has an effective range of over 1,000 yards. It doesn’t have as much energy as the .308, but considering that the projectile is a lot smaller, that’s no surprise, and it will still have more than enough energy for most of what it would be used for.
You wouldn’t want to use it for the biggest game, of course, because the ARC really shines when it comes to accuracy and ballistics, not so much when it comes to stopping power. It has more energy and power than a 5.56, for sure, but it lags behind larger cartridges as you would expect.
The Difference Between Centerfire and Rimfire Cartridges
Rimfire only works reliably with very thin brass, and even with .22LR it’s not uncommon to have 1 or 2 out of every 100 shots have a failure to fire. You’ll only find rimfire on the smallest of rounds, because essentially the firing pin has to dent the rim of the brass hard enough to ignite the primer inside. Larger loads require brass that is too thick for that ignition technique.
On centerfire rounds, the primer is visible from the outside, in the center of the bottom of the casing. The firing pin strikes the primer directly, igniting it and causing a chain reaction that fires the projectile.
The main advantage of rimfire cartridges is that they’re cheap. If you’ve ever compared the price of .22LR or .17 HMR to other loads that are only a little larger, you might have noticed that there is a significant price jump. Larger rounds will always be more expensive than smaller rounds, of course, but there is also a difference in the complexity of manufacturing between rimfire and centerfire.
A world of possibilities opens up when you move to centerfire cartridges. You can shoot out much farther, use much larger projectiles, and overall do a lot more things with them.
Are All AR-15’s .223 and 5.56?
No. While those are definitely the most dominant calibers you’ll see on the AR-15 platform, there are plenty of other options. Some of the most common are 6.5 Grendel, .300 Blackout, and 6mm ARC. The ARC is still very new but adoption is happening quickly. The modularity of the AR-15 platform allows you to use the best AR-15 caliber for your situation.
What Caliber Weapon Is an AR-15?
There are a lot of possibilities here, but most commonly an AR-15 could be chambered in:
This does, of course, beg the question of when does a rifle start and stop being an AR-15, but that is a question for a different article and a wiser man.
How Can I Tell if My AR Is .223 or 5.56?
If you purchased the complete rifle from the gun manufacturer, it will say on the weapon what it is. If the rifle was custom-built and not labeled, you’ll need to find out the model of the barrel and look it up to find out. Everything besides the chamber is identical, and if you have a rifle chambered in 5.56 you’ll be able to shoot .223 just fine. The inverse is not necessarily true, however.
How Fast Does an AR-15 Shoot?
The muzzle velocity for a projectile shot from an AR-15 can vary from 1000 FPS to over 4000 FPS. This depends on what load you’re using and how long the barrel of the rifle is.
If you’re meaning to ask how quickly can an AR-15 cycle through rounds, then the answer is as quickly as you can squeeze and release the trigger over and over. Each time you pull the trigger, the rifle will fire one shot. Then you’ll have to release the trigger completely and pull it again in order to fire another round. The rifle will fire as quickly as you are able to do that.
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!