AS Mr. Brown says, there is no legal minimum thickness and a lot of guns are being shot safely with well under 20 thou mimima. Lightweight Bosses seem to be particularly prone to this, but I know of a pair of Hollands that were recently rebarreled at
17-18 and passed proof handily at that. Someone (I think Greener) did some tests in which barrels were seccessively reamed and shot. It ended when the barrels were so thin they could be cut with a pocketknife. They were still shooting.