Out of proof only means too much metal has been removed. A out of proof gun can be safe to shoot. It can not be sold out of proof in Great Britain. Out of proof guns are reprooofed and then can be sold there. Out of proof means nothing in the US and out of proof guns are sold all the time.
What you worry about is thin barrel walls. Too easy to dent. Thin In the wrong places can be a burst risk by you have to be extremely thin. Worst than thin is reaming 2 1/2” chamber out to 2 3/4”. That area has the highest pressure and no metal removal improves barrel integerty. Worse people go from7 or 8k psi shells to 10k thinking 2 3/4” chambers are automatically saf for all 2 3/4” ammo.
