Have damaged (worn, damaged or badly damaged) and pristine ammo. Split damaged and pristine ammo into small piles and repeatedly combine small ammounts of pristine with 1 small pile of non-pristine ammo until it is all pristine(badly damaged ammo will become damaged, then worn, then pristine.) This can be done repeatedly with most likely unlimited rounds of damaged ammunition, always coming away with larger amounts of pristine ammo. Combining all ammo at once (in the case of pristine vs badly damaged) will average out the pile (35 pristine rounds + 35 Badly damaged come out as damaged.)

I have not tried this with ruined ammo yet, as I did not have any ruined by the bear trap which damaged it. I have only tried this with the shotgun shells and Winchester rounds I had in my pants, as only my pants and boots were ruined. I imagine this effect is produced by the process of continually re-averaging the item health of the smaller stacks which are being combined. I also imagine this works with all ammo, and not just the shotgun and Winchester rounds.