The .300 AAC Blackout (7.62x35mm, 300 BLK) is a rifle cartridge. The purpose of this cartridge is to have an intermediate .30 caliber cartridge in an AR-15 rifle, while preserving the 30-round capacity of standard AR-15 magazines. It also comes in subsonic varieties.
The subsonic bullets weigh in at around 220 gr, while the supersonic round ranges from 110 to 175 gr.
The casing is formed from .223 Remington and 5.56 NATO brass; this means that in a standard AR-15, only the barrel assembly needs to be changed to accommodate this cartridge, because the rim diameter and thickness is exactly the same for both cartridges. It uses the same bolt and magazines as the 5.56.