Drew;
I note in this chart it is stated that shells were fired in a multi-piston gun. No doubt the pressure was recorded at multiple points down the barrel & the curve drawn through these points. I had always suspected that was how the curves were derived, but this chart bears that out. That Ballistite load @ 4.9 tons is really reaching up to the limit, higher than some proof loads.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra