Nine inches is where 300 Blackout was designed to live. The cartridge burns its full charge in roughly that distance, so this build gives up about 150 fps to a 16 inch rifle while losing seven inches of length, and with a suppressor fitted it is still shorter than a bare carbine.
| Component | Specification |
|---|---|
| Chambering | 300 AAC Blackout (7.62x35mm) |
| Barrel | 9 inches, 1:7 twist |
| Platform | standard AR-15, no bolt or magazine change |
| Gas system | pistol length, adjustable block |
| Subsonic load | 220gr, roughly 1,000 fps, 490 ft lbs |
| Supersonic load | 125gr, roughly 2,065 fps, 1,180 ft lbs |
| Suppressed length | roughly 29 inches |
| Intended use | suppressed range work, pest control, home defense |
The reason this build is so popular is the conversion cost. A 300 Blackout upper needs nothing but a barrel swap: same bolt, same magazines, same lower. Nothing else in the AR world is that cheap to change caliber on.
Run subsonics and supersonics from marked magazines and record both zeroes, because they will not share one. Our subsonic versus supersonic guide covers running both from one rifle, the 300 Blackout ballistics chart has the reference tables, and the 300 Blackout ammo shelf carries both.
