Fascinating Thread!!

I think I understand proofing.I assume the conditions to proof are standardized and proof loads should generate at least 30% more pressure than standard.7% more powder should generate at least 30% more pressure

From what I have read a boxlock action is structurally weaker than a bolt action.
How much pressure can each one stand?

From this discussion I assume I should use the table powder charge as the absolute max for doubles even if the velocity is sub par. I assume the powder charge is the limiting factor since pressure is related to the 4th power of the charge used.

This is a big difference from how I load rifle shells. I chrono and attain the velocity desired(compensated for barrel length) and then lower the charge progressively till the most accurate combo is found. Max may be several grains over the powder tables max.
Never had any problems cause I hunt bolt actions in cooler mountainous hunting seasons but I quess that the strength of the bolt action may be more forgiving than a boxlock.

Looks like loading Doubles is touchier with the powder charge and filler et al.Add to that you hunt in hotter climates that cause higher firing pressures