.300 AAC Blackout

The .300 AAC Blackout was developed by Advanced Armament Corporation and introduced in 2010 to solve a problem specific to the AR-15/M4 platform: fitting a 7.62 mm-class bullet into a rifle built around a 5.56x45mm bolt, magazine and bolt carrier, with no parts swap beyond the barrel. Earlier attempts at the same idea, including AAC’s own .300 Whisper wildcat, are effectively the same case; the Blackout is the SAAMI- and C.I.P.-standardised version of that idea.

The case is a shortened and blown-out .223 Remington, cut back to 34.75 mm and opened up to take a 7.85 mm bullet, with a C.I.P. overall length limit of 57.4 mm — short enough to feed from an unmodified 5.56x45mm magazine and to headspace on an unmodified bolt face. That constraint is also what makes the round unusual: with the same case capacity as a 5.56x45mm round but a much larger, heavier bullet, it is loaded two different ways for two different jobs. Light, fast bullets around 110–125 grains give supersonic performance broadly comparable to 7.62x39mm from a short barrel; heavy 220-grain bullets loaded to stay below the speed of sound are quiet enough, especially suppressed, to be the round’s other major use case, one 5.56x45mm cannot match without a different bolt and magazine.

Gaps on this page: no ballistic coefficient is recorded for either load, because the Wikipedia ballistics table this page draws from does not publish one, and no manufacturer product page reachable for free states one either. Neither DVIDS nor Smithsonian Open Access returned a relevant result for this cartridge — it is a commercial and law-enforcement round rather than a standard-issue US military one, so that absence is expected rather than a gap in the search.