M.A.S.S. Gen.2Description Developed and manufactured by Mancraft, this buttstock chassis is a clever and comfortable replacement for standard M4 buffer tube stocks designed to provide a power source to HPA engines. The chassis is CNC machined from high quality aluminum alloy and anodized to minimize scratches and external physical damage. It is installed in place of a standard M4 buffer tube in the AR15 series of rifles and upgraded G36 and AK patterns. The M. A.