The groundbreaking MCR (Mission Configurable Rifle) belt-fed upper receiver assembly is designed to interchange with standard AR/M4 type uppers and readily fits any MIL-Spec lower receiver without permanent modification to the host lower, and is rearward compatible to AR15/M16 models produced as early as 1963. Once installed, the patented MCR upper receiver system can be user-configured in seconds without tools to adapt to virtually any mission profile. Specifically designed for responsible armed citizens, law enforcement and professional security personnel who wish to upgrade their rifle or carbine to MCR performance and capability; standard features include gas-piston operation for extreme reliability in adverse conditions; a 16.25" quick-change (3 seconds) barrel system and a MLOK co-planar hand-guard with rail-interface system for the mounting of optics and modern accessories.