Bear Creek Arsenal BC15 12.7x42 18in Upper Receiver Heavy Mid-Length 1-20 49/64x20 Flash Hide Parkerized. 7x42 is compatible with any mil-spec AR-15 lower. Magazine not included. Specifications for Bear Creek Arsenal BC-15 12.7x42 18in Upper Receiver: Manufacturer: Bear Creek Arsenal Caliber: 12.7x42mm Barrel Length: 18 in Barrel Profile: Heavy Gas System Length: Mid Length Barrel Twist Rate: 1-20 Muzzle Device: Flash Hider Finish: Parkerized Gun Model: AR-15 Handguard Length: 15 in Gun Rail: M-LOK Gun Make: AR Platform Weight: 6 lb Flutes: No Package Contents: Bear Creek Arsenal BC-15 12.7x42 18in Upper Receiver What firearm models does the manufacturer say Bear Creek Arsenal BC-15 12.7x42 18in Upper Receiver is compatible with? The manufacturer says that Bear Creek Arsenal BC-15 12.7x42 18in Upper Receiver is compatible with AR-15Bear Creek Arsenal UAFC4032127X42MHB1812015TNGSR: BC15 12.7x42 18in Upper Receiver Heavy Mid-Length 1-20 49/64x20 Flash Hide Parkerized