If a proof system was established a hundred and fifty years ago many answers would be easier to come by, no doubt. I don't see how establishing one today, would make the gun traders life easier. After all, who would decide what the original barrel diameter was on a 1912 Crescent 16ga?

Our Constitution gave us the right to own guns and the balance of the the Bill of Rights was to guarantee our fight to be left alone. In Europe gun ownership is a privilege granted by the government, it's not hard to see how a proof house could be used to control and register our guns.

No tin foil hat here, just an American who sees more and more governmental infringement on our personal liberty as a bad thing.

Your Mileage Obviously Varies