Starline 300 Blackout Brass Cases (Small Rifle Primer) .300 AAC Blackout, .300 BLK, 7.62x35mm 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. O.A.L.: 1.356-1.36 100 count by weight