Mean Arms Bearing Delay AR-15 9mm 4.5in Upper Receiver Assembly 1/2x28 Black. 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 the surfaces that push the bearings from the inside of the BCG. The bolt face includes our unique center ejector to provide the most consistent ejection of any 9mm AR on the market. The firing pin passes through the ejector. Specifications for Mean Arms Bearing Delay AR-15 9mm 4.5in Upper Receiver Assembly: Manufacturer: Mean Arms Caliber: 9mm Luger Barrel Length: 4.5 in Barrel Twist Rate: 1-10 Threads per Inch: 1/2x28 Handguard Type: Free-Float Muzzle Device: Multiport Muzzle Brake Finish: Anodized Color: Black Fabric/Material: Steel/Aluminum Handguard Length: 4.19 in Handguard Style: Hybrid Rail Gun Rail: Picatinny M-LOK Gun Make: AR Platform Gun Model: AR-15 Weight: 2.5 lb Gas System Type: Proprietary The Bearing Delay Upper Receiver is a complete upper receiver system ready to be paired with the AR lower reciver of your choice Available in 5 different barrel lengths: 4.5'' 7.5'' 11''14.5'' 14.5'' pinned and welded and 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. Package Contents: Mean Arms Bearing Delay AR-15 9mm 4.5in Upper Receiver Assembly What firearm models does the manufacturer say Mean Arms Bearing Delay AR-15 9mm 4.5in Upper Receiver Assembly is compatible with? The manufacturer suggests that Mean Arms Bearing Delay AR-15 9mm 4.5in Upper Receiver Assembly is compatible with AR-15Mean Arms 2101669: Bearing Delay AR-15 9mm 4.5in Upper Receiver Assembly 1/2x28 Black