Tim;
First note the British Ton used here was not our 2,000 lb "Short Ton" but 2240 lbs. Thus a direct multiplication of 3 tons would give 7280 psi. Further note this is not actual PSI but rather crusher pressure. In the modern Shotgun by Sir Gerald Burrard he gave a formula for these British pressures to convert to PSI. This formula was worked out by firing loads in a pressure barrel fitted with both lead crushers & a Piezoelectric crystal to simultaneously record the pressure of a load by both methods.
The formula was said to give a very close approximation across normal shotshell pressures as measured by the British.
The formula is; multiply the tons by 1.5, then subtract .5 ton.
Thus 3 crusher tons would convert to 4 3/8 PE tons. Multiplying by 2240 then gives 9800 PSI. Note this is in quite close agreement to the psi given above by Greg, being within 118 lbs.
I am not totally certain the Low Recoil, Low noise loads actually reach that 11,500 PSI, but Winchester simply is not going to guarantee anything other than they are within SAAMI Specs, which is 11,500 PSI for a 2 3/4" 12 gauge shell.
