The minimum barrel length for the .300 Blackout cartridge, while maintaining supersonic velocity with specific ammunition types, is generally considered to be 7.5 inches. Shorter barrels, such as 4.5-inch and 6.5-inch options, while legal with appropriate NFA paperwork, often sacrifice velocity and may struggle to cycle reliably with certain loads. A 9-inch barrel offers a balance of maneuverability and performance.
Compact barrels are often favored for close-quarters combat, home defense, and confined spaces where maneuverability is paramount. While shorter lengths reduce size and weight, they impact ballistic performance, including velocity, energy, and effective range. The development of the .300 Blackout cartridge itself was driven by the need for a compact, powerful round optimized for short-barreled rifles, specifically within the AR-15 platform.