HSM Ammunition Starline Rifle Brass Unprimed Cases 300 Blackout Caliber 100 Per Bag. 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. Specifications for Starline Brass Unprimed Cases 300 Blackout Rifle Brass: Manufacturer: HSM Ammunition Gun Type: Rifle Caliber: .300 AAC Blackout Primed: No Quantity: 100 Per Bag Fabric/Material: Brass Package Contents: Starline Brass Unprimed Cases 300 Blackout Rifle Brass