A few questions about pistons and tuning?

[look no hands]

Crude but simple. I have no lathe so spin the piston up at slow speed in a drill. Emery applied at the same angle as the seal side and gentle pressure. Any more than that or if spun faster you stand a chance of overheating / melting the side of the seal.
Just a matter of trial and error and hopefully not going to far. Never ruined one yet though.
I try to be very careful around the leading / front / sealing edge.
I've found it best with a spare pair of hands to hold the drill as I'd rather not clamp tools in a vice.
Once the seal is down to the right size (able to slide down with a gentle push) I apply a smear of moly paste to the sides of the seal and wipe off. A small amount of lube is retained in the sanding marks.
I've found the custom air seals very difficult to size as I've found them to be a loose fit on all the pistons so stopped using them and instead stick to O.E. seals.

I use the same method to polish rear bearings on pistons and also to the Diana piston area where the crimps "cloverleaf " it.

I've just put one the new Oz seals on my Brocock Indy (Basically a Gamo action), it's the seal that covers the whole dovetail and reduces the lost volume, seem to fit pretty well but the normal type seal they do does seem a bot loose, I didn't size that one as I think I'll just shoot that one in, it'll be interesting to see what the Maccari seal is like as I've never had one of those before.

Pete