.300 AAC Blackout loading block

The .300 AAC Blackout (abbreviated as 300 BLK by the SAAMI [1] and 300 AAC Blackout by the C.I.P. [2] ), also known as 7.62×35 mm , is an intermediate cartridge developed in the United States by Advanced Armament Corporation (AAC) for use in the M4 carbine . The cartridge yields increased performance in shorter barrels and effective subsonic performance for silencer use when compared to 5.56 mm NATO . The .300 AAC Blackout uses standard 5.56 mm NATO magazines and components with the exception of the barrel.