Here's a great choice if you're planning a .300 AAC Blackout build. This complete upper assembly from Advanced Armament Corporation is ready to drop onto any AR-15/M16/M4 lower with mil-spec push pin locations. It features a 16" nitrided barrel with 1:7 twist, and a Knights Armament URX rail. It is designed for use with a carbine receiver extension, standard GI buffer spring, and carbine buffer.

Specifications and Features:
.300 AAC Blackout caliber
CNC machined aircraft aluminum receiver
16" barrel, black nitride finish
1:7 twist rate
5/8-24 barrel threads
Full auto profile bolt carrier group
AAC 51T Blackout flash hider
Knights Armament URX rail
Black