Proof testing -
Is there any reason - yes, if it blows up it's better where that's planned for and not in a crowd. Sorry to be so blunt. The cannon I saw explode (at 10 or 15 feet away) we (5 of us) were standing behind it (most fortuneately).
You've raised some good questions.
Let's review what we know about metals and stressing them. Somebody's theroem (Poison function?) says that within a range the metal will flex and return to the original shape - a function used extensively for springs. But if flexed past a point, the metal will deform and not return fully to the original shape.
So the question is, how far can we push the metal (what chamber pressure) that assures us of repeated lesser pressure excursions without affecting the strength of the vessel?
When I was in the Artillary, we logged EVERY round fired from the howitizers. The type/weight of the round and the charge. The tube was good for 4,000 rounds at maximum (charge 7). Anything less than charge 7, four rounds were equivalent to one at charge 7. At the end of the life of the tube, the tube was inspected (for cracks at the muzzel) after EVERY round as it approached 4,000 equivalent charge 7 rounds.
WHat margin of safety have we calculated? Should not the tube handle a certain pressure without damage (which we've planned for to be a multiple of what we're going to subject it to).
Should we not be able to subject the tube, without damage, to the maximum calculated pressure it should handle (for proof testing)?
What method of testing should/can we use to determine if the proof round has damaged the tube? Magnaflux? Measuring for stretching?
We should have some confidence in looking to see what is used in practice that survives long term use and abuse.
No answers, just more questions.