BCM Standard 7" 300 Blackout Upper (MCMR-5)

The BCM Standard 7" 300 BLACKOUT upper receiver group pairs a 7" pistol-gas .300 Blackout barrel with BCM's MCMR-5 M-LOK handguard. BCM sells it in its 7" and 9" AR pistol upper line, and its 300 Blackout lineup also includes Standard 9", 12.5", and 16" groups for builders who want more barrel.

The barrel is 11595E spec steel with a manganese phosphate finish, a chrome-lined bore and chamber, and a 1:7 twist. BCM's continuous-taper profile keeps weight near the receiver, thins the barrel out front, and drops the sharp handguard-cap shoulder. The muzzle is threaded 5/8x24, the standard .30 caliber pitch, so it accepts 300 Blackout and 7.62 muzzle devices, direct-thread cans, and 5/8x24 QD mounts. BCM ships it with a 7.62/300BLK flash hider, an MCMR sling mount, and one 3" Picatinny rail section.

BCM states these barrels were designed to run high-velocity ammunition with an H3 buffer, and it recommends running the Crane O-ring over the extractor with 300 Blackout. Plan the buffer and ammunition around that before assuming the upper will cycle every load on a carbine buffer.