Good question, Jim - we know the straigth cases liek the .45's and .50's produce identical numbers for CUP and PSI - ie: 22,000CUP IS the same as 22,000PSI. this only holds true for the straigth cases and does include handgun rounds like the .45Colt, 44mag, etc. - same goes frothose 13,000CUP pressures, being same as 13,000 pounds per squaer inch.

Therefore, we know the various .50 and .45 cases, long and short using those heavy bullets and charges, which are identical to what guys are putting through those inline rifles & what some guys put through flinters and caplocks, are making huge numbers - up to 30,000PSI. that is a LOT of pressure and explains why nipple holds burn out quickly in slug guns.

The high pressure developed by the slugs is due to the friction fit as well as the weight- inertia of getting the bullet to start & keep it moving.

Round balls are much lighter per calibre and therefore produce much less pressure. Note in Lyman's data, the pressures developed with the slugs in each calibre, in comparrison to round balls to see this.