AR-15 Calibers You Need to Know About

AR-15 Calibers You Need to Know About

One of the reasons for the AR-15’s popularity in America is its modularity. AR-15 calibers can easily be changed by simply popping out two pins and replacing the upper receiver.

This ability has supported an industry of many different calibers for AR rifles. Gun buyers and builders have many choices, and you may be wondering about them and which one you should choose.

In this article, we’ll take a look at some of the most popular AR 15 calibers. We’re going to focus only on calibers which use standard AR-15 receivers, ignoring AR-10 and pistol caliber options (for now).


#1 .223 Remington/5.56mm NATO

.223 vs 5.56

The .223/5.56 is the caliber associated with the AR-15 and its military cousins, the M4 and M16. Although the 5.56 NATO is a military version of the .223 Remington, there are minor differences between the two cartridges.

Any rifle that can fire 5.56 NATO can safely fire .223 Remington, as is the case with almost every AR-15 available. Firing a 5.56 NATO round in a .223 Remington chamber can create some complications and even unsafe situations. The latter is almost never an issue with AR-15 rifles but is more common in sporting rifles.

Some rifles may have a .223 Wylde chamber designation. The .223 Wylde is not a separate round in itself, but rather a hybrid chamber that can safely use both 5.56 NATO and .223 Remington cartridge. The .223 Wylde chamber is also considered to promote better accuracy.


The 5.56 NATO was adopted because the cartridges are lightweight and produce low recoil for easy control in rapid fire. A typical .223/5.56 cartridge will shoot a bullet between 50-65 grains at a speed of around 3,000-3,200 Feet Per Second. The lightweight, high-velocity projectile is noted for its ability to tumble, split, and separate upon hitting small targets.

Other bullet selections are loaded from 40-grain varmint rounds to 77-grain long range hunting and shooting rounds. 80 and 90-grain bullets are also sometimes loaded for competition shooting events, but typically are too long to function from the magazine and must be loaded one at a time.


.223/5.56 remains popular in part because it is so common. There are a huge number of guns, accessories, magazines and other parts available. Of all the centerfire rifle cartridges for the AR-15, both .223 and 5.56 are among the cheapest available. At the time of this writing, brass-cased .223 and 5.56 cartridges can be frequently bought in bulk for 30 to 40 cents a round – while other cartridges can easily cost double that for factory ammo.

The low recoil is another reason many people gravitate towards this caliber. The semi-automatic operating system of the AR-15 combined with the lightweight, small caliber round makes for very low recoil. This makes it easy to stay on target and makes it easy for anybody to shoot it safely and effectively – regardless of age or physical status.

.223 rounds are sometimes chosen for security and self-defense purposes because of their propensity to tumble and come apart when met with interference. When combined with a rapidly expanding bullet construction, .223 bullets are more likely to lose energy moving through walls and other barriers, presenting less threat of collateral damage if the target is missed.

The very high velocity and lightweight bullet also tend to create wound channels much larger than the diameter of the bullet.


The .223 bullet is considered insufficient by many hunters and is not legal for deer hunting in some states because of its low diameter.

The lightweight bullet and tight bore mean that a long barrel is required to fully burn all the gas from the gunpowder. Shorter barrels leave more unburnt powder, resulting in low bullet velocities and excessive noise. Short barreled 5.56 rifles are known for incredible muzzle blast and comparatively poor ballistic performance, especially since the bullets rely on speed for terminal effects.

Although the speed of the bullet is impressive at the muzzle, the light bullets quickly lose velocity and are susceptible to the effects of wind and other deflection. Shooting at long range with the .223 or 5.56mm gets challenging as the bullet quickly loses steam and gets pushed around by the effects of the wind – although this disadvantage can be an advantage as this makes it a great trainer round learning long-range shooting in the wind.


#2 .300 Blackout


The .300 Blackout started life as a wildcat known as the .300 Whisper (or the .300/.221 Fireball before that) but is more commonly known as the .300 Blackout today.

The .300 Blackout is based on the Remington .223 case, with the neck expanded to take a .30 caliber bullet. It is designed to use both supersonic bullets (faster than the speed of sound, like most rifle bullets) and subsonic bullets (slower than the speed of sound, resulting in no sonic crack when fired).

Because of its similarities to the .223 Remington, an AR-15 can be converted to fire .300 Blackout cartridges by changing only the barrel.

Typical supersonic .300 Blackout loads typically fire a projectile between 110 and 150 grains, while subsonic projectiles are much heavier at 200 grains and above.

Supersonic loads have comparable performance to the popular 7.62X39mm round or the .30-30 Winchester used in popular lever action rifles. Subsonic loads are near silent when fired from a suppressor but have less energy – a little more than a .45 caliber handgun. The real benefit of the cartridge is the ability to switch between the two in the same gun without any major adjustments.


The .300 Blackout is designed to take advantage of standard AR-15 parts but provide a round that performs better in shorter barrels. The .300 Blackout effectively burns most of its powder within 9” of the barrel, so it is ideal for short-barreled AR pistols and carbines.

The heavier bullet is much more capable for close range hunting while maintaining the low recoil of the .223 Remington. The .300 Blackout has become a particularly popular round for brush hunting animals such as hogs, deer and coyotes. 

The cartridge is optimized for shorter barrels, meaning that short-barreled defensive firearms do not lose their ballistic capability and do not develop the muzzle blast that short-barreled 5.56 firearms do.

Commercially available subsonic rounds mean that the rounds can be easily suppressed and very quiet, useful for both hunting and home defense.

The .300 Blackout has decent commercial support for quality factory ammo, but reloaders can also take advantage of common .223 Remington brass to make their own cases.


Because of the fat, heavy bullets and low speeds, the .300 Blackout does not tend to do well at long range. It is optimized for shots less than 300 yards with supersonic rounds and inside 100 yards with subsonic rounds. Longer shots are certainly possible but require extra skill and calculation. 

Although many manufacturers support commercial ammo, it is still more expensive than .223 or 5.56.

It should also be noted that .300 Blackout can cause a catastrophic failure if accidentally fired in a .223 chamber. While it’s true that ammunition is always dangerous if loaded in the incorrect firearm, .300 Blackout can create a perfect storm for failure. The case is short enough that the bolt can close fully with the bullet giving a false headspace on the chamber shoulder. With a bullet much larger than the bore, the pressure backs up and will quickly blow apart an AR-15 receiver. This isn’t a reason not to use the caliber – but something to make note of if you plan on swapping between calibers frequently. It’s also a good argument for labeling or color-coding magazines since both calibers will use the same mags.


#3 6.5 Grendel


At one point, the 6.5 Grendel cartridge was the intellectual property of Alexander Arms and could not be produced without a license from the company. Although the capabilities of the round were well noted, this kept some manufacturers from adopting the cartridge. SAAMI standardization and the ability for other manufacturers to freely use the chamber specification allowed more widespread adoption and use within the industry.


Designed to extend the range of the AR-15 past 1000 yards, the 6.5 Grendel was one of the first commercially successful long-range AR 15 cartridge. Using 6.5mm cartridges with high ballistic coefficient, the 6.5 Grendel retains much of its velocity at long range. It is a very stable long-range cartridge, with less drop and wind drift at long ranges than many full powered rifle cartridges.

6.5 Grendel shares a bolt face diameter with the 7.62X39mm round, commonly fired in AK pattern rifles. Although it requires a different bolt and barrel, a complete upper receiver will still drop on to a standard AR-15 receiver.

The most common load for the 6.5 Grendel has it pushing a 123-grain bullet about 2,600 feet per second. Although heavyweight bullets are most popular for long range shooting because of their ballistic advantages, bullets as light as 90 grains are sometimes used for hunting.


The chief advantage of 6.5 Grendel is its performance at long range. Out to 700 yards or so, it has a nearly identical performance to the venerable .308 Winchester (which requires a full-sized rifle larger than an AR-15). It does this with roughly half of the recoil energy and a lighter cartridge.

The bullet weight and energy put it in a class capable of regularly killing small to medium game at moderate distances, although there have been reports of animals as large as elk being killed at longer distances.

Although factory hunting and match ammo for the 6.5 Grendel can be expensive like many other rounds, there is another option. Wolf Military Classic is a line of inexpensive, lacquered steel cases. The line has been expanded to include 6.5 Grendel with a 100-grain FMJ bullet, which is available for less than 30 cents a round! Although you probably won’t win accuracy competitions at this price, it makes the 6.5 Grendel a much more economical cartridge to shoot.


The 6.5 Grendel is a well-rounded cartridge with relatively few glaring weaknesses, aside from it being a specialty round.

The unique case head diameter means that part commonality is limited, which is not a huge deal breaker. The case also feeds more reliably with 6.5 Grendel specific magazines, which generally sacrifices a small amount of capacity versus a standard 30 round mag. 


#4 .224 Valkyrie

Like the cartridge proceeding it, the .224 Valkyrie was designed to extend the reach of a standard AR-15 rifle. Built off the case of a 6.8mm SPC (another popular AR-15 cartridge), the .224 Valkyrie uses a bullet the same diameter as the .223 Remington. However, the shape of the cartridge allows for more powder and for the seating of long, heavy bullets with high ballistic coefficient.

The .224 Valkyrie was only very recently introduced by Federal as a new sporting cartridge, but it has very quickly seen industry support. In short time, it has come to be a capable round for long range shooting with very low recoil and is seeing use in both semi-automatic and bolt-action rifles.

One of the big claims to success for the Valkyrie is its stability at distance. The bullet stays above the speed of sound for 1,300 yards or further. This is important because a bullet going transonic (dropping below the sound barrier) experiences disturbances and becomes unstable, losing significant accuracy.


As stated, the .224 Valkyrie has great long-range ballistics. It is quickly becoming a popular sport shooting round, especially in competitions such as the Precision Rifle Series and in long-range 3 Gun matches.

The caliber has very little recoil impulse, allowing users to spot their own shots more easily at extended range. It is also easier to shoot because of this and is gaining consideration as a hunting round for young shooters. Although .22 caliber is traditionally small for hunting, the Valkyrie takes advantage of comparatively large case capacity and long, heavy bullets which transfer much more energy to the target.

Although new calibers can be expensive and hard to find, Federal planned the launch of the .224 Valkyrie with industry support from rifle, ammunition, and accessory makers. Ammo was available in hunting, match and training loads from launch, and support is continuing to grow.


At this point, the round is still considered something of a specialty cartridge and is new to the industry. Although it seems to have taken off and gained popularity, it’s yet to be seen if the cartridge will remain popularly supported in the long term.

For best feeding, the Valkyrie really requires a specialty mag designed around 6.8 SPC ammo. Although these are readily available, it is something of a drag if you have lots of .223 magazines you’re already hoping to use.


#5 .458 SOCOM

If the AR-15 has a reputation for being a small-bore rifle, it’s probably because many people haven’t heard of the monster .458 SOCOM.

Designed after conversations of the 5.56 NATO’s inability to stop enemy combatants with a single shot, the .458 SOCOM was built to revamp the standard M4 carbine with a deadlier cartridge.

The SOCOM was specifically designed to be able to use very heavy subsonic bullets with standard AR-15 components. Specifically, it shoots super heavy 500 and 600 grain bullets, while supersonic bullets range from 200 to 450 grains depending on the load.


The .458 SOCOM was specifically designed to use standard AR-15 components for easy replacement. A complete upper receiver will drop easily onto a standard lower, and a large majority of the components used for the upper receiver are also standard components. This even includes the magazines!

The SOCOM will feed from standard 5.56mm magazines, specifically those designed for military use with M4 and M16 rifles. Because they feed single stack instead of double stack, capacity is reduced – roughly 7 rounds in a 20-round magazine or 10 in a 30-round magazine – but most standard AR-15 magazines can be used with no modification or issue.

Like the .300 Blackout discussed above, the SOCOM can cycle subsonic bullets for very quiet shooting. However, the bullets it uses are 2-3 times the weight of .300 Blackout subsonics, so the round packs a massive punch. Its power is often compared to the classic .45-70 Government round, which was used to kill Buffalos in the Old West. The subsonic capability combined with the massive weight bullet makes the SOCOM a favorite of hog hunters, who want the ability to quickly put down a large and aggressive animal.


As stated above, magazine capacity is limited because of the size of the case. As a hunting round, this isn’t too big a deal – especially since many states place a limit of 5 rounds in a hunting magazine anyways.

Long range ballistics of the SOCOM aren’t stellar even with the lighter weight bullets – meaning you’ll have much more to contend with to take a longer shot. That’s not the focus of a cartridge like this anyways, so it’s to be expected.

Perhaps one of the biggest shocks with the SOCOM is the price of ammo. Loaded factory ammo (subsonic or supersonic) typically floats between 2 to 3 dollars per shot – roughly 10 times what a single shot of .223 Remington costs. Of course, you can always reload, but it’s still going to be more expensive due to the unique brass and the cost of the components involved.

So, the SOCOM probably isn’t going to be your new plinking or go-to rifle. But, since it uses standard AR-15 components, you might think about buying just the upper in case you have a hog problem or just want to lob some big bullets.

A Whole Wide World

As stated earlier, there is a whole world of AR-15 wildcats and specialty cartridges out there. To write about them all might require a small book. By narrowing the list down to 5, I hoped to give an idea about some of the substantial offerings out there right now that you might come across when buying. There were many others that could have been on this list but ultimately didn’t make the top 5 cut.

Are you curious about a certain caliber? Want to see a more expansive article like this? Leave us a comment so we know what you’re looking for!

In the meantime, do you have a favorite specialty cartridge? Just like to stick with the standard 5.56 NATO? Leave us a comment and let us know!


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