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The MK2 upper receivers increased rigidity limits barrel deflection which aids in consistency/accuracy. A gas expansion chamber is machined above the charging handle track, combined with dual vents machined at the rear (each side, above charging handle latch) help to redirect and vent gas away the shooters face. Included with the BCM Upper Receiver Group is one 3" Picatinny Rail Section and one BCM MCMR Sling Mount.
Complete Upper Features:
Note: These barrels were designed to run with a carbine buffer with subsonic ammo and a H3 buffer with high velocity ammo. Fine tuning may be required in limited instances based on ammo and buffer choices that work best for the desired application. BCM also recommends running the Crane O-ring provided with all BCM Bolt Carrier Groups over the extractor with 300 Blackout.