Alternative AR15 Calibers
One of the greatest features of the AR platform rifle is its modularity and ability to interchange parts. From a buyer’s perspective, this is a great way to maximize the value of the platform. Whether you are bored with the tried and true .223/5.56 and just want to try something new, that’s okay! AR15 calibers aren’t like wives - you don’t have to keep just one… You can try as many as you can find and keep the ones you like.
In fact, there are really only three limiting factors when it comes to AR15 caliber conversions: First is the cartridge length, as cartridges that are too long won’t fit into a magazine or chamber safely. The second limiting factor is the bolt head diameter of .480 - which is about as large as you can go before there are issues. The third limiting factor is your budget! Beyond these three factors, there is a myriad of options available for shooters looking to customize their AR15 with an alternative caliber.
While we specifically call out barrels, bolts, and magazines as necessary conversion parts, keep in mind that your rifle might also need new a buffer systems depending on how your existing rifle is tuned.
So which calibers are best? Well, that’s up to you to decide but we’ve compiled a list of some of the common (and a few less common) alternative AR15 calibers to consider grouped by their typical purpose.
Long Range Shooting
If you’re looking for velocity, you’ll be hard-pressed to find something that fits the bill better than the .22 Nosler. Capable of surpassing 3,500 ft/second, the .22 Nosler isn’t a Ferrari. It’s an F-15. Not only is the .22 Nosler fast, but it’s also ideal for precision, long-range, and work out past 1,000 yards. Plus, as a proprietary design of Nosler, a company known for its precision work, you know that you’ve got a reliable round.
Upgrading to the .22 Nosler is relatively straightforward. All of your standard AR15 parts will work with two exceptions. In order to upgrade your AR15 to run with the .22 Nosler cartridge, you’ll need to upgrade your barrel and your magazines. While neither options are super cheap in comparison to a standard AR15 part, it certainly beats having to purchase a whole new BCG. In addition to the magazine and barrel, though not required, shooters would also probably want to look at upgrading their gas system to accommodate the hotter round.
The 224 Valkyrie was created to compete with and outperform the .22 Nosler and focused on being a viable cartridge for military and law enforcement use in both close quarters and long-range engagements.
The 224 Valkyrie was designed as a necked down 6.8 SPC bullet, for speed and power, with a 60-grain bullet capable of hitting 3,300 fps and puts out a generous 1,594 ft-lbs of energy at the target.
While many AR manufacturers offer factory options, shooters looking to convert their AR to 224 Valkyrie would need a 6.8 SPC bolt and magazine, and a 224 Valkyrie barrel of course.
The 6mm ARC was designed by Hornady, a brand known for the quality of its precision ammunition, to be a long-range tack driver wrapped up in an AR platform. Designed for the DOD and field tested by Special Operations, the 6mm ARC has earned a reputation for its high velocity and long-range knockdown power, with many shooters stating they’ve achieved 1” MOA groupings at 1,000 yards + out of an AR assembled for long range precision.
With a wide range of bullet weights, available velocity and energy will vary, but you can expect around 3,500 fps to 2,800 fps depending on whether you go with a 55-grain bullet or something in the 100-grain + range.
To run the 6mm ARC you’ll either want to purchase a factory option or swap our your barrel, magazine, and bolt.
Hitting Hard Close Up
At the opposite end of the spectrum from .22 Nosler and 224 Valkyrie, there is the .458 Socom. Designed for close quarters, as opposed to long range, the .458 Socom is based on the 50 A&E cartridge and fires a .458 projectile. The result is MASSIVE energy dispersion upon impact. The .458 Socom excels when used with SBRs in conjunction with a suppressor.
While one could technically get by using 5.56 magazines with the .458 Socom, it is truly a better idea to use .458 specific magazines. Users will also need to upgrade their bolt, barrel, and consider upgrading their gas block, gas tube, and buffer spring. You can also get an upper with a larger ejection port just to be safe - so yeah best bet is just get a new upper in 458 SOCOM.
If you’re looking for unrelenting, raw power, the .50 Beowulf is where it’s at. Capable of chewing up and spitting out most glass, foliage, armor, etc, the .50 Beowulf is a monstrous round. However, in addition to the increased cost of ammunition, the .50 Beowulf requires a 7.62x39 bolt, a new barrel, and at minimum new followers for your magazines. In reality, you’ll want to make sure you have a mid-length gas system, .50 Beowulf-specific mags, and probably a new buffer and buffer spring.
Inspired by the legendary Jeff Cooper, created by LeMag Firearms, and licensed to Bushmaster, the 450 Bushmaster packs a punch and was designed for maximum energy in a close-range scenario. If you had to bring one gun to a bear fight, you’d probably want to consider the 450 Bushmaster in an AR platform.
The 450 Bushmaster was designed as a hunting round. While it might double for self-defense, the intended use was for large game where a straight walled cartridge is required. Anything at the other end of this round is facing 250 grains at roughly 2,200 fps. The end result is some serious energy upon impact starting at 2,700 ft-lbs at the muzzle.
Conversions for the 450 Bushmaster require a new bolt, barrel, and magazines, although you may be able to modify your 5.56 mag followers to convert the - though for reliability reasons that is not recommended.
.300 AAC Blackout
If you’ve ever wanted 7.62x39 style ballistics out of an AR15, look no further. Much like the .458 Socom, the .300 AAC Blackout was designed for CQB scenarios and excels in a short barrel with a suppressor. However, the .300 AAC Blackout is somewhat unique to our list because of the widespread availability of affordable ammunition and the ease in conversion.
One of the contributing factors to the widespread success of the .300 AAC Blackout is that it requires only a barrel change for a conversion. The .300 AAC Blackout is 100% compatible with standard AR15 mags and bolts. That makes it one of the cheapest conversions available.
Plus with the large variety of subsonic ammunition, this round is perfect for shooting quietly.
This is not to say that any of the other calibers can’t be used for hunting - they can, but these calibers are particularly well suited for hunting.
When picturing Bill Alexander’s 6.5 Grendel, just picture a chubby 5.56 round with roughly the same velocity but more knockdown power. Although we should note, the parent case for the 6.5 Grendel is the 6.5 mm PC.
Of course, those aren’t the only benefits of the 6.5 Grendel when compared to the 5.56. You’ll also get a flatter trajectory, better wind resistance, and a more sustained rate of accuracy out to about 800 yards. As with most calibers, results will vary depending on the exact bullet weight and powder used. However, it’s these characteristics that make the 6.5 Grendel ideal as a hunting round.
Builders looking to convert their AR will want to change out the barrel, barrel extension, and bolt.
Developed as a project between Remington and the Army, the 6.8 SPC was originally designed to increase the lethality of the AR platform in combat. However, when it failed to gain acceptance in the military it picked up popularity as a hunting cartridge.
While the 6.8 SPC suffers from reduced velocity in comparison to the 5.56 at 2,460 fps at the muzzle compared to 3,060 - it does pick up an increase in transferred energy boasting 1,612 ft-lbs compared to the 5.56’s 1,289. This makes it an ideal caliber for hunting varmints and medium-sized games. Additionally, the 6.8 SPC is able to be utilized in states where 5.56 does not meet the minimum dimension for taking deer.
As with most alternate calibers, simply swap your bolt, barrel, and magazines, and you’re ready to lock and load.
The 350 Legend has a complicated history, steeped in the rules and regulations surrounding states that require deer hunting to take place with a “straight-walled” cartridge. Winchester, identifying the need, took up the project to create the 350 Legend which has become a popular new deer caliber.
Rocketing a 150-grain bullet at 2,325 fps and providing a whopping 1,762 ft-lbs of energy at the muzzle, the 350 Legend provides hunters with everything they need, and nothing that they don’t and its mid-level recoil makes it ideal for smaller frame hunters.
For shooters wishing to build or convert their existing AR, keep in mind that the 350 Legend will need a different barrel, bolt, and magazine than a standard .223/5.56 AR.
Most people have never heard of this wildcat round but the concept makes a lot of sense. Take a standard 223 case and neck it up to a .277 / 6.8 mm-sized bullet. This is exactly what they did with the 300 BLK but with a .308 / 7.62mm bullet. The benefit of this round over the 6.8 SPC is that you don’t need to change your bolt or use special magazines - all you need to do to run the 277 Wolverine is swap your barrel. The drawback is less velocity and therefore less terminal energy - everything has a tradeoff.
The AR15 platform has long been hailed as one of the modular platforms on the market, and this caliber list certainly proves it. From hitting hard, to reaching out to long distances, to hunting big game and dishing out some quiet “pews” - the AR15 has proven that it can do it all.