Is this even possible, given the premise that metal has been moved beyond it's elastic limit to spring back?

If the barrel's metal has indeed moved physically past it's eleastic limit, it should be a very simple matter of measuring to find out how much ie: before and after. A 1" micrometer capable of measuring to the ten-thou will easily show change. Actually, most of these mic's are easy to read to the 1/2 ten thou. on the rotating spool (.00005").