The smallest practical 300 Blackout. At 7 inches you are past the point of diminishing returns on velocity, but you gain the shortest package the cartridge can produce, which is the entire point of a personal defense weapon build.
| Component | Specification |
|---|---|
| Chambering | 300 AAC Blackout |
| Barrel | 7 inches, 1:7 twist |
| Platform | AR-15 pistol or SBR |
| Gas system | pistol length, adjustable block strongly advised |
| Supersonic velocity | roughly 2,100 fps with 110gr |
| Subsonic velocity | roughly 995 fps with 220gr |
| Velocity cost vs 16 inch | roughly 250 fps supersonic, 30 fps subsonic |
| Intended use | vehicle, concealment, close quarters |
Two honest warnings. Cycling gets sensitive at this length, particularly with subsonics fired unsuppressed, so an adjustable gas block earns its cost here more than on any other build. And unsuppressed muzzle blast is genuinely unpleasant, so this configuration really wants a can.
Note that subsonic loads barely notice the short barrel, losing only about 30 fps against a 16 inch rifle, because they are capped below the sound barrier by design. The full barrel length table is in our 300 Blackout barrel length guide and the ballistics chart.
