This is only related so far as proving goes but many years ago I spent 3 months turning up a nice length of modern steel into a 30" 8 bore barrel as a hook breach ML. I then had the black powder proof load details from the Birmingham Proof House for all gauges from 4 to .410. I was so confident of the steel and the contour and weight of the barrel that I decided to prove it as a 6 bore rather than as 8. This was to give an added level of safety when using the standard 8 bore loads. I marked the barrel in 4 places and miked it and recorded each measurement - (done in the white before fitting to the stock) loaded it with a 6 bore provisional proof load, tied it to a large tree and fired it from a distance. Re measuring, clean up inspection with a magnifying glass and good light showed no changes at all. I was now free to commit more time to fitting and finishing the barrel. Once finished the gun was remarked in the same places along the barrel, re miked, loaded with a 6 bore definitive proof load, tied into an old truck tire down at the local dump and fired with a long string. Again same result, no visible or measurable alteration, or signs of stress. This was a muzzle loader proved to breach loading cartridge proof loads, I was wery happy with the result and the gun performed very well with standard 8 bore hunting loads.