Sir you and I are in agreement. The process of proving does not allow anybody to abuse a firearm or subject it to unknown pressures.

It does however because the proof pressure is 30% greater than the service pressure of factory loads set the benchmark for safety.

In answer to Gathumper's query. Yes it can cause your double to fail, but it is far better on the bench than in your hands. The day I was there and having mine done I was told that a few days earlier a .600NE had been forced off face by the process.
To guard against this if you are going to build your own you must look carefully at the action you are going to use. A cheep second hand, loose light 12 guage, is not perhaps the best place to start.
A new magnum 10 guage is going to give you more margin. Now it will be a mite heavier perhaps, but that is why our homebuilts are metaphorically the drag racers of the double gun spectrum whilst the best guns are the F1 racing cars. Everything about them is the culmination of years of experience trading strength for weight and balance.
H+H could build my .450#2 a pound lighter than I could and not sacrifice anything in strength. I could not. But then I could not afford to trade off the Ł 90,000 it might have cost me. For my circumstances no ammount of excess ugly weight is worth Ł 90,000. Unfortunatly my ex wifes lawyer and the divorce judge did not agree with me.

Also consider carefully which cartridge it is to fire. The reason mine is in .450#2 NE is because it is 13 long tons pressure rather than .450NE which is 17 long tons nearly 25% less thrust for the same balistic performance.
Do the calculation correctly and it will not fail.

Regards