Starline Brass Star300BLKEU: The .300 Blackout is based off of a shortened 5.56x45mm case that is necked to .30 caliber. For AR-15 platform rifles and pistols it only requires changing the barrel to go from 5.56 to .300 Blackout. The Blackout\'s main purpose is to be used at subsonic velocities in suppressed firearms. Many bolt action and single shot rifles are now being produced in this caliber as well.