New guns action is messing up casings....?

DO NOT SAND IT! You're on the right track though, what you want to do is polish it, not sand it. Get a piece of chore boy copper scrubby and spin it in the chamber using a cleaning jag and a cleaning rod. Just spin it between your palms back and forth, clean out any dust, chamber a shell and check for scratches, keep doing this until the scratches on the shell do not appear. This will remove the burr, and not affect the inside diameter of the chamber. Sanding will increase the inside diameter.