Reinforced Version II Gearbox for the following Marui series: M4/M16, MP5 & G3. Super strong gearbox was engineered by auto mechanics who have a deep understanding of the dynamic relationships between pistons and gears. This gearbox is made with extra thick walls and high density alloy for maximum performance. Tests showed reliability even after 200,000 rounds using an M190 spring configuration for 800fps of output. Anodized non-slip finishing. Uses standard Philips type screws for easier installation instead of the torx screws as found on Marui gearboxes.