Miller is dead on and he saved me getting out my calculator and doing a lot of writing. The force (back thrust) is calculated as the sq" in area times the pressure. This is something that folks need to know when considering making a double rifle out of a shotgun. Also in a rifle (dry) chamber the expanding brass grips the chamber walls and reduces the back thrust. The English at one time calculated chamber pressure in rifles using a greased chamber.

MP Sadly Deceased as of 2/17/2014