Hi, I have just read a thread on piston washer sizing !
I have a well used Bsa lightning, It has a Gas ram which I have pumped up to give an output of 11.6 Fpe. ( .22 )

I know nothing about piston sizing, but when I took the piston out, The parachute seal looked OK but was a loose fit, Looser than I would have expected ?
The gun shoots VERY well & has a great trigger , ( All the reports on these say they are not so good )

I have made a silencer for it, & its pretty quiet, put it up for sale soon as I am finished with it, & 2 gas rams .22 & .177.

But this is my real point, I loaned the Gas Ram to a mate of mine, He has a Lightning also & said it fired well, However he was only getting 11.2 Fpe,??
His gun is in nice condition, better than mine, Same pellets, Etc.
He checked his & mine, His was still 11.2 & mine still 11.6 Fpe

I can see a problem for the gas ram or indeed spring makers if 2, almost identical guns can vary so widely,
Thats probably why the ram outputs are low !
Cheers, ged.