easy way to get a lot more power and a nicer shooting gun, it to convert the piston to take a diana 28mm parachute type seal.
* remove the piston head and buffer and put them in the parts bin
* shorten the piston nose spigot a mm or two
* thread it M8 or similar
* make an ally adapter (dovetail type) to suit the diana seal, including a shoulder of around 2-3mm (you can't make the stroke too long, or it'll run out of cocking slot)
* screw the ally seal adapter onto your M8 threaded spigot and fit the seal
will make 11+ FP all day long with a very moderate spring, and shoot softer too (better piston cushioning / more swept volume)
Takes less than an hour.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.