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.
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