300 Blackout, also designated 300 AAC BLK or 7.62x35mm, was specifically designed to fill a void created by 5.56 NATO: high terminal performance and reliability in a suppressed semiautomatic carbine. One common complaint of 5.56x45mm is that it performs poorly at subsonic velocities, which are required to get maximum noise reduction when shooting a suppressed weapon.

The speed of sound is about 1000 to 1100 fps, depending on elevation, temperature, and humidity. Standard 5.56x45mm ammunition has a typical muzzle velocity of about 3000 fps; the velocity is needed to make the small, light bullets effective against enemies. When the small, light bullets of 5.56x45mm ammunition are slowed to subsonic velocities, the bullets have little stopping power and the ammunition may not create enough pressure to cycle a semiautomatic weapon.

To address these issues, 300 AAC BLK subsonic ammunition uses heavy, streamlined projectiles travelling at just under the speed of sound. These bullets, typically 200 gr or more, have excellent terminal ballistics - they make large holes (almost) all-the-way-through enemies. The pressures required to drive such heavy bullets to near-sonic velocities gives plenty of energy to cycle semiautomatic carbines, making 300 AAC BLK subsonic the ideal round for use in suppressed semiautomatic weapons. The typical subsonic bullets, with a high BC from their shape, retain velocity down range: 220 gr Match King bullets with a muzzle velocity of 1050 fps is still going 950 fps at 300 m, making a 300 m hit equivalent to a .45 ACP at point blank.

300 AAC BLK ammunition is also available in supersonic loads, with bullet weights normally in the 120-130 gr range. These rounds are accurate and have excellent terminal performance, regularly out-performing 7.62x39mm in both categories.

In either super- or sub-sonic loads, 300 AAC BLK is based on the 5.56x45mm cartridge, allowing the use of M27 links for belt-fed weapons, and standard 5.56x45mm magazines in carbines. Converting from 5.56x45mm is typically just a barrel change!