Doug, 17.5 tons would be 17.5 times 2240 which would equal 39200 psi.

This is based upon a British long ton being 2240 lbs. So 1 ton per square inch of proof or service pressure would be 2240 pounds per square inch. However, it gets confusing attempting to convert the old system of British Proof pressure to the radial piezo transducer system we widely use today. Actually, there is no direct conversion calculation.

I'm not sure what the standard operating pressure of a .222 rimmed would be, but I expect it would be over 39,200 psi since the .222 Remington operates at around 46,000 psi. I found a Thread on the Enfield-Rifles.com forum that has some pretty good information. The first Thread in particular has a copy of a 1967 American Rifleman article that explains how the British measured their service pressures for rifles in the late 1800's

https://www.enfield-rifles.com/british-system-of-chamber-pressure-measurement_topic10711_page1.html

My question about how one would determine a safe operating pressure for these old single shot actions comes from HalfaDouble's earlier remark about the .22 Hornet Martini conversion that had the cartridge head back through a soft breech block. I suppose a lot of the information we do have for various actions came from actual Proof House proof testing. And in this country where we do not have government controlled Proof Houses, much of what we know apparently came from experimentation (or trial and error method) by many of the old wildcatters and rifle builders like Jerry Gebby, C.S. Landis, Harvey Donaldson, Hervey Lovell, etc. There is also information found in some of Frank DeHaas's Single Shot Rifle books concerning action strength, different designs, and suitability for conversions to different cartridges.

A true sign of mental illness is any gun owner who would vote for an Anti-Gunner like Joe Biden.