Build an AR-15: AR Calibers
Originally written by TOM MCHALE on JULY 1, 2015
The AR platform can be adapted to dozens of different calibers, like this Rock River Arms LAR-6.8 chambered in 6.8 Remington SPC.
In these troubled times where everyone wants to stir up trouble and hate over our differences in appearance, I want it to go on record that I don’t just tolerate diversity, I embrace it.
In my safe right this moment, I have AR-type rifles chambered in .223/5.56mm, 6.8 SPC, 300 Blackout, and .308 Winchester. Of course, .308 is admittedly related to the Armalite AR-10, not AR-15 family. When it comes to diversity, I put my money where my mouth is. As we speak, I’m Jonesing for something in 6.5mm. Not only that, I’ve been begging the folks at Smith & Wesson to come out with an M&P 338, chambered in, you guessed it, .338 Lapua. Why? Because diversity is fun.
One of the limitations for caliber swaps is the length of the standard magazine and magazine well.
The neatest and spiffiest thing about AR-type rifles is that they’re not really just a single rifle type. They’re a platform, or set of common design specifications on which you can build a whole lot of different, but related, guns.
The split receiver design of AR-type rifles lends itself to using a standard lower receiver with stock, grip, magazine well, and fire control system containing trigger, hammer, sear and safety mechanisms. Then, in theory, and for the most part practicality, you can mount different upper receivers that house the chamber, barrel, bolt carrier, bolt and hand guard. Of course, you can’t just put any caliber upper on a standard lower. The primary reason for this is that the magazine well is part of the lower receiver, and that creates some limitations on cartridge size. For example, one of the big differences between AR-15 and AR-10 type rifles is the larger magazine well on the AR-10 to accommodate .308 Winchester. Everything is bit larger owing to the need to handle bigger cartridges.
.223 Remington / 5.56x45mm
What’s not to love about the “original?” Well, more or less the original. Back in the ArmaLite days when the AR direct impingement rifle was being developed, the original plan was to make it a .308 / 7.62mm rifle, but the military folks liked the smaller caliber option. The thinking was that the average soldier could hump a lot more cartridges if they were smaller and lighter.
The debate over terminal effectiveness will outlast all of us, so I’ll stick to the theoretical performance idea. A very small and very light bullet, moving insanely fast, should fragment, tumble, and wreak all sorts of havoc on organic targets. There’s plenty of evidence that it does. There’s also plenty of evidence that it’s not a magic one-shot stopper like the old 7.62 and .30-06 ammo fired from M-14s and M1 Garands. More suppressive firepower weighed against better terminal effectiveness per hit? You make your call as to your personal preference.
One thing that has become interesting is the standard caliber AR platform’s suitability for home defense. That same theory that says a light and fast bullet will fragment and tumble also helps with potential over-penetration in the home. If you want to perform a fun science experiment, take a bunch of wallboard to the range, separate it to create artificial walls, and shoot it with 55 grain .223 ammo and some normal handgun hollow-points. You might be surprised at which one penetrates the most interior walls.
.300 AAC Blackout / .300 Whisper
I love this caliber in an AR platform. Before the hate mail starts, I’m not claiming it’s “better” than any other AR cartridge, I just really enjoy it, mainly because I love to reload ammunition. The 300 AAC Blackout allows for a near-infinite variety of bullet weight and velocity combinations.
The huge 300 AAC Blackout subsonic projectiles are stuffed into cut down .5.56mm cartridge cases.
You can launch a 245-grain hunk of pure lead downrange at 1,000 feet per second. This flying brick approach yields 544 foot-pounds of energy. Using a suppressor, this is a freakishly quiet combination sure to elicit very un-macho giggles from anyone present at the range.
You can also go supersonic and zing an 110-grain bullet at 2,500 feet per second. This .30 caliber projectile cranks out 1,527 foot-pounds of energy. That’s about 50% more kinetic energy than a standard .223/5.56mm 55-grain bullet moving at 3,000 feet per second. It’ll stay supersonic out past 600 yards or so depending on local conditions.
As an AR-type rifle cartridge, I love that it fits in standard .223 / 5.56mm magazines. That’s because it uses cut down cartridge cases from .223 / 5.56mm. I don’t mean vaguely similar, I mean it’s literally the exact same case, trimmed down and reshaped with a lower and less aggressive shoulder to accommodate a .30 caliber bullet. There are some 300 Blackout specific magazines that do a better job of handling odd-shaped bullets, but, for the most part, compatibility is excellent. It even uses the same bolt carrier and bolt as the 5.56mm.
Use of shorter barrel ARs prompted development of alternate calibers like this 300 Blackout to gain improved terminal performance.
While the AR-type rifle in traditional calibers is assumed to be a 400 yards or less cartridge, some enterprising folks have turned a variant into a pretty nifty longer range performer. Bill Alexander and Janne Pohjoispää developed this round as a way to make a standard AR rifle reach out to 800 (and more) yards. It’s designed to fit in a standard AR magazine size, so it gains its extra oomph by using a larger diameter case to launch a long and thin bullet.
Most 6.5 Grendel bullets weigh between 90 and 140 grains, with a common one landing at 123 grains and moving at over 2,500 feet per second. The low-drag projectiles maintain supersonic velocity past 1,000 yards. It also carries over 1,000 foot-pounds of kinetic energy to nearly 450 yards.
Bolt action 6.5 calibers can outperform it, but only because they’re not limited by the same overall length restriction due to the magazine well. You’ll want a longer barrel to take full advantage of this one, and, due to the larger case diameter, you’ll need a new bolt. Even though the cartridges are larger, you can still stuff about 26 of them into a standard size magazine.
6.8 Remington SPC
Here’s another one I really like, although I can’t exactly say why. If “just because” is a good enough reason to like an AR variant, then I’m sticking with that reason.
The 6.8 Remington SPC was developed by some special forces guys, some other folks from the Army Marksmanship Unit, along with some engineers from Remington. The idea was to provide better terminal performance than the 55 and 62 grain 5.56mm NATO ammo being used overseas, especially from short-barreled rifles. Tired of evil d00dz not falling down when shot; they wanted a heavier and larger diameter projectile that could still work in the AR platform and maintain reasonably high-capacity magazines.
The 6.8 Remington SPC cartridge is based off the .30 Remington.
While the overall length is the same as 5.56mm, so it fits in existing magazine wells, you’ll probably need 6.8 SPC specific magazines for best reliability. That’s what I’ve found anyway. The cartridge case is not based on the 5.56mm, but rather a .30 Remington, so to convert a standard AR, we’re talking about replacing bolt, barrel and, magazine.
While 6.8mm sounds cool and metric, it’s really just a .270 caliber bullet, which actually measures .277 inches in diameter. Because we gun people like to make things confusing, I guess. If you’re a hand loader, .270 bullets are plentiful, even in these times of limited reloading supplies availability.
The “average” 6.8 SPC load contains a 115-grain projectile moving at 2,640 feet per second. Of course, you can load lighter or heaver bullets too. The average load translates to 1,785 foot-pounds of kinetic energy. Our average bullet will stay supersonic out past 850 yards, and bullet drop is still manageable at that distance by many magnified optics, so it has more “reach out and touch” capability than the lighter weight 5.56mm rounds.
There’s only a little bit of cheating happening with inclusion of this one on the AR variants list. Yes, you can use a standard AR-type lower receiver with the AR57 upper to fire the tiny little FN 5.7x28mm cartridges.
However, it’ll look somewhat non-traditional. The FN magazines for this cartridge, used by the PS90 carbine, store cartridges sideways and rest on top of the upper receiver. The magazine well on the AR lower is actually the ejection port. By the way, this unusual magazine holds 50 rounds.
Ou example cartridge is the SS195LF. This little 28 grain bullet hums along at 2,350 feet per second and delivers 344 foot-pounds of energy.
The AR57 upper receiver actually uses the top-mounted FN PS90 magazine and ejects through the lower receiver magazine well.
If you’re looking for the ultimate in AR diversity, how about one that accepts it’s traditional “enemy’s” ammo? The 7.62x39mm cartridge has been used in at least 319 trillion AK-47 type rifles worldwide for over 2,000 years. Ammo and magazines are plentiful to say the least. Besides, the 7.62x39mm offers a heavier bullet and more kinetic energy than the 5.56mm.
You can get an upper receiver to handle the comm-block ammo, just be aware that you may have to resort to some tinkering and Dremel tool use to get it humming. The usual reliability problems center around three areas. The AR-type rifle magazine well is straight, which doesn’t always sit well with the tapered cartridge and associated banana clip design of AK magazines. Others have reported problems with feeding related to low gas pressure and have resorted to drilling out the barrel’s gas port. As for the Dremel? The standard AR design uses a split feed ramp which tends to hang up the larger 7.62x39mm cartridges. Industrious shooters have put Dremel to metal to smooth out the feed ramp area into a single ramp.
Or, you can cheat a little bit and pick up a rifle “like” an AR that’s optimized for the cartridge. CMMG’s Mutant and the Rock River Arms LAR-47 both offer lower receivers built specifically for the AK cartridge fan club.
If you favor lobbing cinder blocks down range, you might consider one of the standard AR rifle big boy caliber options, .458 SOCOM.
Like other non-traditional AR calibers, this one was developed after complaints of 5.56mm stopping power. This cartridge swings to the polar opposite end of the slow and heavy versus light and fast scale by using a lengthened .50 AE cartridge case and 300 grain bullet moving at 1,900 feet per second. That generates 2,405 foot-pounds of kinetic energy.
The beauty of this one is near complete compatibility with the standard AR, excepting of course the barrel. You don’t want to try and stuff a .458-inch bullet into a .224 inch hole. Everything else, magazines, bolt, buffer and springs, is the same. [Correction: The bolt is not compatible, that’s a typo!] The big fat cartridges stack singly into a standard AR magazine, so a 30-round 5.56mm magazine ends up holding 10 rounds of .458 SOCOM.
Can you say “infinite bacon?”
.308, 300 AAC Blackout and 5.56mm cartridges.
Not to be outdone by tiny little 300-grain, .458-inch bullets, Alexander Arms offers complete rifles and uppers in .50 Beowulf. This one is intended to replicate the famous .45-70 Government round.
If you go with the 400 grain bullet option, that’ll lob down range at 1,800 feet per second and deliver 2,878 foot pounds of energy.
Pistol Cartridges: 9mm, .40 S&W, .45 ACP
Too many to list here, numerous makers offer upper receivers for standard pistol calibers like 9mm, .40 S&W and .45 ACP. If you want the handling and easy sight picture of a carbine, maybe one that matches your handgun caliber is in order? The longer barrel length will give your pistol rounds a little more velocity, and therefore longer effective range.
Coincidentally, .22LR has a number of ways to fit into the AR-rifle family. The bore size is pretty darn close to the standard .22LR version anyway. Regardless of the .223 Remington name, the standard AR rifle uses bullets that are .224 inches in diameter. Regular .22LR ammo varies a bit, from .223 to .224 inches in diameter, so you can shoot it through a standard AR barrel with reasonable success. As a result, a bolt and magazine change, combined with a chamber insert, can turn your existing rifle into a .22LR plinker. Of course, dedicated .22LR upper receivers and complete rifles are plentiful too.
The standard AR barrel is close enough in diameter to shoot .22LR ammo through it with a conversion bolt.
In the “why the hell not?” category is the .410 shotshell upper from Safir Arms. The upper connects to standard AR-type lower receiver and lets you whale away with .410 shot shells or slugs. The magazine is proprietary as you might imagine. You can get magazines in 5, 10 and 13 round versions. Why 13? Maybe the better question is, why not?
In the “Because ‘Murica!” category is the Zel Custom – AR-15/M16 Tactilite upper receiver. This, like most of the others mentioned here, mounts to a standard AR-type lower receiver.
Th .50 BMG cartridge is just a hair longer than the 5.56mm, so there’s not a snowballs chance in hell that one will go through a standard magazine well without melting it first. As a result, this upper receiver is a single shot only, and you load the .50 BMG cartridge through a bolt action in the upper.
A Smith & Wesson M&P 15 VTAC (top) and Colt M4 chambered in .22LR (below)
And many, many more…
In a quick and very informal count, I found at least 53 other chamberings for the AR-type rifle, ranging from 5.45x39mm to .30 Carbine. Oh, that M&P 338 I mentioned earlier? ArmaLite makes an AR-10 variant called the AR-30A1, chambered in, you guessed it, .338 Lapua Magnum.
Celebrate diversity? Heck yeah! But ask yourself this question: What do you want this rifle to do? If you’re an experienced shooter, and this won’t be your first AR, then the sky is the limit. If you’re just getting into the black rifle game, consider the benefits of sticking with the tried and true. Nothing wrong with the old 5.56.
Stay tuned for other articles in this series here on our online ar15 builders forum or,
you can read the original post as well as others in the series:
https://www.gunsamerica.com/blog/build- ... roduction/