The MCU wire harness is intended for use with REV3 FCU's that feature the built in MCU (Motor Control Unit) for running the replica's box magazine. This model features the standard A&K connector such as used with the A&K M60 and PKM series so that no soldering or splicing is required. Simply plug it in and go! Supports both priming (constant) and switched (trigger activated) modes of the box magazine.