Hi Ed,
so it can be kind of like a trade off then. Definitely something to think about. Thank you.
SPUN TRUE and STRAIGHT either mounted on piston or a fixture in a LATHE is best. @ 220 grit wet/dry on a flat backer & slowly sanding away the seals lip as it spins at nearly the same angle seal is made with. Ideal fit is when seal still offers resistance to move in & out within compression chamber, yet tight enough piston won't fall under it's own weight ... tho close.
Professional method ....
SPUN TRUE and STRAIGHT either mounted on piston or a fixture in a LATHE is best. @ 220 grit wet/dry on a flat backer & slowly sanding away the seals lip as it spins at nearly the same angle seal is made with. Ideal fit is when seal still offers resistance to move in & out within compression chamber, yet tight enough piston won't fall under it's own weight ... tho close.
Professional method ....
Seal sizing is trial and error. What motorhead said is absolutely correct, BUT! you want to start a little too large, and slowly work your way into the sweet spot. Take too much off too early, and the seal is toast. Sizing a seal takes multiple tear-downs and re-builds, until you get it right. A long time ago, before I bought a chrony, I did this all by FEEL. How does the shot cycle feel? Too tight, and it'll bounce less, but you won't get the MV you're looking for. Too loose, and the gun will hammer. I tend to leave my replacement synthetic seals on the larger end of the spectrum, and let them run in.
Which brings me to leather seals. Granted they are not up to the higher energy of today's higher powered spring power plants, but the absolute elegance of a leather main seal is that by simply shooting the gun, the seal will conform to the compression tube. The more you use it, the better it becomes. Someone before me once said "A proper air rifle consists of steel, hide and timber."
@jiminpgh
I figured this would be an exercise in caution. If it comes down to it, I feel a little more empowered from the response.
Glad you brought the leather seals into the light. Way back I bought an RWS 34 that had one. As new, the pellet would only shoot hard enough to piss a sparrow off. Upon disassembly I found the seal dry as a popcorn fart. I didn't know the first thing about the characteristics of the leather so I simply rubbed it over well with whatever RWS seal lube that was available at the time. The difference was night and day and yep, got only better over time.