Lesser Known AR-15 Calibers
The following article and photos is from the fine folks of Primary Arms Content Marketing Team
The AR 15, first developed by Eugene Stoner over half a century ago, has become one of the most widely
adopted and successful firearms in modern history. Most of the time, we see it in its standard
chambering, .223 or 5.56mm NATO. But, over the long and storied history of the AR platform, there
have been a lot of interesting calibers that have been tried.
For each of these unusual calibers, we’ll start by briefly introducing it and giving some of the reasons
behind its development. From there, we’ll talk about intended uses for some of these, which can get
both interesting and highly specific.
.50 Beowulf
.50 Beowulf is, as the name implies, a .50 caliber round for the AR platform. To shoot this 12mm
behemoth, you’ll need a whole new upper receiver, as is common with these unusual calibers, but, it
uses standard AR magazines.
The 50 Beowulf is a .50 caliber round, yes, but it’s a short one, using the same overall length as standard
5.55mm NATO. As far as intention goes, this is a special tool meant to stop vehicles that refuse to stop
at checkpoints. It was first developed and saw limited adoption in Iraq and Afghanistan in the US’
involvement in both nations increased with the Global War on Terror.
.300 Blackout
A relative newcomer to the scene, the .300 Blackout has become one of the most popular AR cartridges
of the last several years. It was developed in response to a more or less perennial search since the
adoption of the M16 in the 1960s for a larger caliber version that would suppress well.
The .300 blackout does just that, working spectacularly well with subsonic rounds and making for a
suppressed AR package that leaves the 5.56mm in the dust in terms of noise reduction.
9mm
While it’s coming back now into fashion with the concept of a pistol caliber carbine, there have been
9mm ARs since the very beginning. Known as the AR9 in some circles or as the Colt SMG635 in others,
folks have been using ARs in 9mm since the 1960s.
The main benefit to a 9mm AR, usually, is its compact size when compared to a standard M16, especially
in the era when 20” barreled guns were the norm. Thus, tankers, aircrews, drivers, and some people on
executive protection details have gone with 9mm ARs over the years.
One of the big critiques of the 5.56mm is that it often takes more than one round to end the threat
posed by an enemy combatant. While this was well known by the end of Vietnam, it took Operation
Gothic Serpent, also more commonly known as Blackhawk Down, to get the Army’s attention.
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