Variant Chamberings for the AR Platform
Dr. John Woods 12.15.15
Believe it or not, the AR-15 rifle platform comes in a wide variety of cartridge chamberings besides the .223/5.56. I highly suspect that the standard .223/5.56 chamber comes in probably 90 percent plus of the ARs manufactured. However, this should do nothing to deter lovers of the AR configuration to actively try out other viable chamberings in an AR rifle.
To date, and perhaps this statistic is outdated, but the AR-15 rifle currently can be acquired or fitted with 4 different rimfire calibers, including the .17HMR, .17 Winchester Super Magnum, the .22 Long Rifle, and the .22 Winchester Magnum Rimfire.
There are 17 centerfire calibers available in the “inch” measurement category in an AR-15. The most well-known would be the .17 Remington, .204 Ruger, .243 and .25 Winchester Super Short Magnums, .30 Carbine, .30 Remington AR, .300 AAC Blackout, .300 Whisper, .450 Bushmaster, .458 SOCOM and the .50 Beowulf. If you are not familiar with some of these chamberings, then run a Wikipedia search on them.
Next there are the centerfire calibers in the metric measurement category. There are an additional ten chamberings in this group. Popular choices here include the 5.45x39mm (aka the AK-47), the 6.5 Grendel, 6.8mm Remington SPC, the 7.62x40mm Wilson Tactical, the 9mm Parabellum, and 10mm auto pistol rounds.
So total we are talking an additional 31 chamber options in the AR-15 Platform. In many cases all that is needed to change from the .223/5.56 to another caliber is to swap out the upper units on a standard lower unit. Sometimes in cases like the .300 AAC Blackout and .300 Whisper, all that is needed is the upper with the use of the exact same .223 magazines.
In other cases a complete new rifle is needed for the round to function as a complete rifle. These might include the 6.8 SPC and the .458 SOCOM. I certainly like the idea of just buying an upper unit and quickly slapping it on top of an existing lower and going to work.
Of all these additional chamberings, I have shot many of them, but personally I use the 6.8 Remington SPC in a Ruger AR and the .300 AAC Blackout (along with the .300 Whisper which are basically interchangeable). I use the 6.8 on wild hogs, and have successfully used the .300 Blackout on white-tailed deer.