XC - sorry for the slow response - Southern SXS and such.

I was not aware that more than one proof load was fired per barrel. As for the testing theory, it would depend on how close you wnted to come to failing something on any given proof shot. It could well be that two slightly lower pressure loads would be better than one higher pressure load for detecting weak guns without smashing them. I would assume that the proof houses worked out a long time ago how high the pressure, heavy the load, and number of repetitions needed to non-destructively verify that any individual gun was not likely to bite the hand feeding it, abuse barred, of course. St. Geo, isle thereof, spawned some might fine engineers who could have easily cyphered this out.

Note that the WW M-21 proof load marathon showed that it took a considerable number of loads to disable any gun and none of the tested guns "burst" as far as I have been able to find out.