Called the 300 BLK for short, the .300 AAC Blackout is a 7.62x35mm rifle cartridge. It was developed by the Advanced Armament Corporate (in the United States) for the M4 carbine. It was designed to employ standard AR-15 magazines while achieving ballistics similar to the Soviet 7.62x39mm cartridge.
300 BLK should not be used in a rifle that’s chambered for use with 7.62x40mm Wilson Tactical or .223/5.56 cartridges. The 300 BLK was designed to meet the special needs of certain military operations. It was found that certain military operations called for better performance than standard high-energy rounds provided while they also needed subsonic performance that was greater than 9mm could provide. That’s what sparked the 300 AAC Blackout’s development.
In cooperation with Remington Defense, AAC developed it to meet all the goals the military was after. Since then, the 300 BLK cartridge has gained a good amount of popularity among many different groups. For example, in 2011, SSG Daniel Hormer used it to win his 4th USPSA Multi Gun Championship.
The 300 BLK is also used abroad in both the Netherlands and the United Kingdom. In July 2017, the UK opened a five year (plus five years optional) contract to get an on-going supply of 300 BLK, while the Netherlands requested 195 cartridges chambered in 300 BLK for the Dutch Maritime Special Operations Force in July 2015. In the USA, many defense teams have requested conversion kits so they can move into using more 300 BLK in their missions.