The Bearing Delay Upper Receiver is a complete upper receiver system ready to be paired with the AR lower receiver of your choice. Available in 5 different barrel lengths: 4.5", 7.5", 11", 14.5", 16" - all threaded for a 1/2-28 muzzle device. No 9mm-specific buffer needed. The Bearing Delay Upper Receiver is designed to work with standard AR-15 buffers. The heart of the Bearing Delay Upper Receiver is the unique ball bearing bolt carrier group (BCG). The ball bearings interface with corresponding features in the barrel extension and provide the lockup in battery and the delay after firing. The system can be tuned by replacing the Lifter to change the angle of Mfg: Mean Arms