Airsporter S good and bad ?

[El Garro]

I've been messing about with piston head conversions to enable parachute seals to be fitted and have seen some surprisingly different power levels between the 2 S's and 1 mk 5 I had
Ignoring the mk 5 which was a different design that had leaky joints, the S I've been working on at the mo will not go above 10.5 I realise there isn't much swept volume in them to start with but as I've managed to get a few more mm stroke by fitting the adaptor (could go more but need to make sure the cocking link has clearance and more importantly that the under lever drops down enough to be able to get your fingers around it before it starts compressing the spring) but still no real improvements. The shooting cycle is a lot better as there's no metal to metal contact at the end of the cylinder, I use a double o ring buffer with locating channels machined into the head to keep them in place and the piston is lined. I've opened up the groove on the parachute seal to make it more flexible and 'parachutey' yet power is still low. The tap seal is good too. No leaks. I've tried a few different springs including everyone's favourite, the trusty ox which have given the best results. Maybe as it's short on the stroke, a punchy spring works best ? Who knows
One thing has crossed my mind. I've noticed a lot of variation in dimensios on the standard airsporter so the big question is.....
Are there good and bad S models
Any ideas on that gents ?

Edit, just thought to add, it's a .22 mk 3 S and it was thputting out 9 with the old spring and o ring head.
Tried various pellets too.