300 Blackout transforms the standard AR-15 into a hard-hitting, suppressor-friendly platform that thrives in both supersonic and subsonic loads. Arm or Ally offers 8.3″, 10.5″, 12.5″, and 16″ barrels machined from 4150 CMV steel and treated with nitride or FNC to guard against corrosion in high-humidity suppressor environments. A 1:5 twist stabilizes 220- to 240-grain subs, while a 1:7 option maximizes velocity with 110- to 125-grain supersonics for deer-legal energy. Pistol- and carbine-length gas systems are ported for reliable cycling with low-back-pressure can designs; many barrels ship with pre-dimpled gas-block seats and factory-installed thread protectors on 5/8-24 muzzles. Round out the build with an adjustable block, H2 buffer, and our house-brand nitrided BCG and you’ll enjoy whisper-quiet operation without sacrificing mag compatibility or bolt life.