This is a AEG M4 build with a stock gearbox that was completely torn down and upgraded. I upgraded the gearbox with 1:13 gears and I got a high torque motor for it. This gearbox has a new piston, piston head ,cylinder, cylinder head, and a SYSTEMA SPRING M120 making this rifle a 395-405 fps gun. For external parts I purchased a suppresser, outer barrel, red rail system, ch11.1 lipo battery (with charging station), red dot sight, flashlight, angled grip, and 2 mags in total.