Managed to strip the Model 50, cut off another three coils & rebuild it yesterday evening, with a plastic spin washer at either end of the spring. Before there was about 3mm between the coils due to preload, now there is 4mm. Out of the gun there is 6mm between the coils.
Shot it three times and the power is a measly 6.6 ft/lb s
However it's cockable now, almost on par with a 97, it fires with better manners too. The only thing I can think of now is more piston weight, ideally a bit of steel tube the same shape as the spare cocking slot & tack welded in place would be good, but I'll maybe add a few washers instead in the short term.
For now I'll let it bed in & concentrate on making the fore-sight raiser so It can come to the bash.
Only three days of work next week and then it's bash time