Designed in conjunction with PWS & FM Products, the Brownells BRN-180(TM) Complete Upper Receiver is inspired by the ArmaliteA(R) AR-180 of the 1960s. Developed as a successor to the original AR-15, the AR-180 was a civilianized version of the AR-18, itself an upgrade on the M16 platform. The BRN-180(TM) incorporates many of the AR-18/AR-180 improvements - including its robust and reliable gas piston operating system. (Yes, the originators of the AR-15 / M16 eventually embraced a piston!) The BRN-180(TM) Gen2 Upper Receiver Group improves upon the original BRN-180 with two main upgrades. First, all BRN-180(TM) Upper Receivers now ship with an Adjustable G Mfg: Brownells