That's the chart I was referencing, Drew.

I eyeballed the 9" point which is where this all started and guesstimated 2 tons.

That's a whopper of a proof load, and if the pressure curve is anywhere near correct the strength of blackpowder proved guns has been underestimated for generations.

Perhaps this is the chart that gave rise to the idea that black maintains higher pressures down barrel longer than smokeless.

"The price of good shotgunnery is constant practice" - Fred Kimble