Hi everyone ! Recently, I've got a nice 500 from a antique shop.But when I tried to knock the piston unit,I've ruined the barrel.Now I will buy a 400nn brown tortoise parts to replace my 500. Now I have two solutions here 1. Knock the 400nn piston unit out and replace with the 500 piston knob (is it compatible?) 2. Just change the cap and become a 500nn Any another better methods? Thanks