A rather old gearbox by today's airsoft standards, ASTKilo23 goes over the Systema M4A1 M120 Gearbox... "I recently picked up quite an oldy, a complete Systema gearbox from Norwest Airsoft, a small Airsoft shop here in Spokane. It seems to have been used a bit, but I open it up none the less to see what the quality of 10+ gear old parts and tuning are like."