There is no MWT rules per say. Guns with MWT of .015-.020" pass proof. Proof is lost when internal barrel bores are altered. Like making the chambers longer, adding screw in choke tubes, honing the barrels so much that they become enlarged by .010". Striking the barrels is external to the bore and should not take them out of proof. I think it is a odd distinction but it does make some sense. Proof just declares the barrels can withstand a load and are still on face with no damage afterwards. They did not blow up, bulge or suffer damage to the action or locking.
Dents get removed, which cause the barrels to need re-striking and refinishing. If you watch closely, many guns end up at auction after having a dent removed but not yet re-blacked. The owner finds out how thin they will be and decides to dump them. They are still in proof and hence still legal to sell. But they now have thin barrels and are more of a dent risk than before. Thin barrels, in good shape will give good service and be safe to shoot, with proper pressure loads. Hence my never ending issues of what is safe and how to make safe shells. Worse, with the spot shortages of powder it seems like when ever I find a safe, proper load that powder suddenly becomes unavailable for the next two years.
I have several guns with thin barrels and I pay much more attention to where the dent is than just how thin they are. A thin area 20+" from the breach is one thing and one at 10" is a very different thing. Pressure is great, the closer to the breech, and my fingers never venture 20" out but do 10" out. So it is how thin and location, location, location. I bought a very nice hammer gun several years ago which had thin areas that were in the choke areas, 28" out, on 30" barrels. One barrel was .012" after some previous owner had the choke removed. Why you needed to alter the chokes on a gun which came with cylinder I can not figure. Perhaps he was removing pitting. Anyways the barrels measured .024+" in every other spot but did have a very thin muzzle. I sold the gun and explained the risk was much great for denting a muzzle with such in muzzle areas.
