The 300 Blackout (300 BLK) was designed by Remington/AAC to create a reliable, compact .30 caliber round for the AR platform that uses a standard bolt and magazine. 300 Blackout is optimized for suppressed fire with heavy bullet subsonic loads but can also be used with supersonic ammunition when extra range is needed. The twist rate will work well for subsonic and supersonic loadings alike and there is typically no need for an adjustable gas block when switching back and forth between the two loadings. All these specifications come together to create a round that is great for home defense, law enforcement / military use, as well as hunting.
The 300 AAC Blackout delivers ballistic performance comparable to 7.62×39, yet possesses a base diameter that optimizes functionality in an AR-type platform. It is cross-compatible with 223 Rem/5.56×45 magazines. The 300 Blackout’s .30 caliber bullet can be made especially long, and as the result may weigh up to 220 grains — quadruple the weight of the typical 223 Rem round’s. So heavy a bullet conveys a subsonic muzzle velocity and correspondingly softer report. Converting an AR-15 to fire 300 Blackout is as simple as swapping its barrel. Although functionally identical to the 300 Whisper, the 300 AAC Blackout is its own SAAMI standardized cartridge.
The .300 BLK (SAAMI) or .300 AAC Blackout (C.I.P.) was developed in 2009 and introduced in 2010 by Advanced Armament Corporation (AAC) at the request of the US Military. The initial goal was to develop a round that offered better performance when suppressed that was designed for use in the M4, and the result required a simple barrel swap to run the new caliber with all other components of the rifle carrying over.
The .300 BLK is used primarily for defensive or military purposes. Militaries have adopted the cartridge for use within their Special Operations commands and as Personal Defense Weapons because of the reliability when using suppressed firearms. It is also used for some medium game hunting.
Because of the similar cartridge sizes between .300 BLK and 5.56x45mm NATO/.223 Rem, care must be taken to not chamber the incorrect ammunition in firearms. Staff Sergeant Daniel Horner of the US Army Marksmanship Unit used a .300 BLK to win his 4th USPSA Multi Gun National Championship in 2011.
The .300 BLK is a rimless, bottleneck cartridge with an overall length of 2.26” and a case capacity between 24 to 25 grains (H2O). It fires a .308” diameter bullet using small rifle primers with a maximum pressure of 55,000 PSI. Standard rifling twist is 1:8”. The cartridge was designed to fit inside existing AR15 pattern magazines, though some aftermarket magazines have internal geometry that won’t fit the larger bullet.
Ammunition options for the .300 BLK offer subsonic and standard velocities with bullet types being full metal jacket, hollow point, polymer tipped, open tip match, and barrier penetrating in weights ranging from 110 grains to 220 grains depending on need or mission.