The .300 AAC Blackout (designated as the 300 BLK by the SAAMI and 300 AAC Blackout by the C.I.P.), also known as 7.62×35mm, is an intermediate cartridge developed in the United States by Advanced Armament Corporation (AAC) for use in the M4 carbine. Its purpose is to achieve ballistics similar to the 7.62×39mm cartridge or, even more similarly, the 7.92×33mm Kurz cartridge in an M4 while using standard M4 magazines at their normal capacities. Ammunition in .300 BLK cannot be used in a rifle chambered for 7.62×40mm Wilson Tactical. It is mainly derived from the .300 Whisper concept, but differs in having been submitted to the SAAMI.
While 5.56×45mm NATO has enjoyed widespread acceptance in military circles, the nature of the missions encountered by some special operations groups often demand a round that provides better performance than that available in the high-energy, standard velocity rounds, and subsonic performance greater than standard 9mm (the ubiquitous pistol and submachine gun) round.
To meet this demand, AAC developed the 300 AAC Blackout in cooperation with Remington Defense. The new cartridge was intended to negate many of the perceived drawbacks inherent to other large caliber cartridges used in the M4. Colt Firearms and other arms makers had previously chambered AR-pattern rifles and carbines in various .30 caliber rounds but encountered problems. In the case of the 7.62×39mm, its relatively severe case angle caused feeding issues unless specially modified AK-47 magazines were used, and even then results were unsatisfactory. Modified bolts were also needed owing to its larger case head diameter. Rounds such as the 6.8 SPC and 6.5 Grendel had similar parts-interchangeability issues but did allow for the use of the standard M4/M16 30-round magazine, albeit with a reduced capacity.