it's a systema gearbox (originally mp5 ) with a systema full tune up kit. I put in a cnc air nozzle, a trigger and selector plate, all for m4 by lonex so now it's a m4 gearbox, its a $300 gearbox and a $200 tune-up kit, both are discontinued not to mention the lonex parts, it's still brand new, I want to go back to hpa for the adjustability from field play to cqb play. hit me up with offers, I'm interested in guns, hpa units and cash (it has an m110 spring not the m150, so it's 330-390 fps depending on barrel length so you don't have to worry about 500 fps but I have tons of springs I'll ship it at any fps you want)