The "Imperial" (Long) ton equals to 2240 lbs. By simple multiplication 3 & 3¼ tons psi would relate to 6720 & 7280 lbs psi rather than 6,000 & 6,250 we would think of using the US 2,000 lb "Short Ton".
Note also these are "Crusher" pressures & should not be confused with actual PSI. Many, Many years ago the British ran some tests with a pressure gun set up to simultaneously record both Crusher & PE (actual psi) pressures from the same shot. A series of these led to the result it was stated that for practicl purposes a formula could be used "Within normal shotgun pressure ranges" of 1.5T - .5tT = psi.
Working through this formula for the 3 & 3¼ ton loads
gives psi readings of 8960 & 9520 respectfully. These PE reading are thus a full Ton (2240lbs) "Higher" than the respective crusher pressures. Also note I have "Nothing" at all to indicate this is applicable to any systems of recording pressures other than the British one with which the tests were run.
Though I have no exact knowledge of the fact, at the time these tests were run they were likely done using a bulk powder such as Schultz or EC. It is entirely possible, even probable, that like tests run with powders of varying burn rates would produce varying results.
In Short, There simply is "NO" reliable method of converting crusher pressures to True PSI. Not this formula, Not adding 1,000 lbs or any other simplistic method.
