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.
* Starline Brass does not sell any loaded ammunition. What you are purchasing are brass cases only and will need to be loaded into ammunition in order to be fired from a firearm.