Proofing was usually done with at least two loads, an initial overload followed by a smaller overcharge. Proofing is a trickier business than you might think, as you have to walk a fine line between ensuring that there are no flaws in the steel that will cause catastrophic failure without overstraining the steel and perhaps starting a crack that will cause failure down the road (the secondary proof used historically was to guard against that possibility). On top of that, the primary concern historically was to weed out any barrels with a bad weld, whereas with barrels made from modern steel bar the primary concern would be a flaw in the steel as it came from the foundry, such as a cluster of sulfide stringers or a bit of lead that wasn't distributed evenly throughout the steel. There has been quite a bit of discussion on the subject here over the years, if you want to do a search on barrel proofing.
I like the idea of proofing a barrel in the abstract, but I'm not convinced that I (or most people) are capable of doing it without running the risk of doing more harm than good or 2) that historical proof standards (and those still used in Europe) designed for welded-up barrels are the best means of testing modern barrels.
The US military still proofs its barrels, BTW. They use an overpressure cartridge delivering 70,000 psi to test barrels designed for 55-60,000psi operating loads (5.56 NATO), followed by particle inspection. Since that standard has been designed with modern construction methods in mind, that might be a good place to start. The problems are the difficulty of getting a particle inspection done and the difficulty of determining an adequate proof pressure for a given barrel wide variability of operating pressures in a muzzleloader.
