[ithacaartist]

Something thin to take up space might also a be a bead of silver solder, or one laid down by micro-welding. Unless I'm mistaken, peening can squish again. And I agree that making a shim that thin would be next to impossible. Extending or adhering something permanently to the bearing surface of the flange, then facing it off on a lathe until it's just right would be my first choice. Loc-Tite might also do the job by itself if it clocks close enough. I've heard that 1/8 turn with the wrench setup after the hand tightening bottoms out is about right. If the barrel will hand tighten beyond proper position, though, I'd nix the Loc-Tite only approach.