Proof?
Jack, Mark, Drew; I think Jack's recent post frames my concerns rather precisely. Jack and Mark cite British proof of 3 1/4 and 3 1/2 Tons. If those are tons made of English pounds, and expressed as the force on English square inches, they are pressures of 6500 psi and 7000 psi. Metric tons and English square inches raise those numbers 10%. If they are metric tons and metric centimeters squared, they are about 15000 psi, comparable to SAAMI proof loads.
Measuring transient pressure, especially when the total event occurs in milliseconds, is not a precise experiment; the numbers quoted always have to be referenced to the time that the pressure is averaged and the measuring instrument.
That is the reason I prefer to choose a gentle load from a simple shot column length and shot speed. I know that a shot column 1 inch long leaving the gun at 1300 ft/sec is pretty hot irrespective of gauge; a shot column 3/4 inch long leaving at less than 1200 ft /sec is pretty mild.
I think RST tables back me up on this; 10 ga 1 1/8 ounce (.686 inch), 12 ga 7/8 ounce (.603 inch ) and 3/4 ounce (.520 inch), 16 ga 7/8 ounce (. 732 inch) and 3/4 ounce (.626 inch), 20 ga 3/4 ounce (.672 inch)and 28 gauge 5/8 ounce (.756 inch) all at less than 1200 ft/sec are without a doubt very gentle loads, and it is not necessary to quote pressure to verify this.
Incidentally, Parker, prior to the formation of the interindustry for runner of SAAMI in the 1920's, proofed by lengthening the shot column (adding more shot weight, not powder weight) according to TPS.
Best, Austin
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