.300 Blackout

**.300 AAC Blackout** (often "300 BLK") is a .30-caliber cartridge designed from the start to run on the AR-15 platform from short barrels — including suppressed. It was developed by Advanced Armament Corporation in 2011 to give the AR-15 platform a round that would cycle reliably from barrels as short as 9", push .30-caliber projectile weights, and transition cleanly between **subsonic** (heavy bullet, suppressor-friendly, hearing-safe) and **supersonic** (lighter bullet, comparable trajectory to 7.62x39) with only a magazine change. It does not replace 5.56. It occupies a specific niche: the suppressed short-barreled rifle. Outside that niche, other cartridges do what 300 BLK does more cheaply. Inside it, nothing else comes close.