Hello, i have some 32/20 brass i want to use for a 310 Martini and the rims are too thick ,what is the best way of thinning them to fit ? method Thankyou for any advice

If you take metal off of the face of the case the primer pocket will be too shallow.
You have to take off the front of the rim.
A lathe will do it, or a file with a safe
side. I think you can buy the correct cases.

I believe that Bruce Bertram makes cases for the .310 Cadet...so I guess that it would be easier to get them direct from him?

If you are set on doing it you can either, as HAWKINS says, use a lathe to trim down from the front.

Or make a die (piece of steel pipe hardened and with an internal rim recess turned in it) and use a hammer to swage the rims down by slipping the die over your 32-20 case and hitting the die with a hammer as the case rests on a steel surface.

This will squash the rim to the desired thin profile with some overspill.

Then just put a wood dowel in the case mouth, tap the case from the swage die and leaving it still on the dowel spin in a drill chuck against a file to trim off the excess.

If you have a lathe that method, using a lathe, turning down the face, is probably quicker!