I had the same problem with my KWA gearbox recently, except it was caused by me forgeting to loctite my modify piston head to the piston and it breaking loose. The KWA gearbox is NOT a standard V2 their new 2GX gearbox is coming out soon, another option is to contact them through their help desk at
http://support.kwausa.com/access/una....kwausa.com%2F They may have some shells around the shop that they can part with.