I truly wish I could now recall the source to give proper accrediation but I read many years ago from a source citing a major ammunition makers ballistitions as saying in shotguns the pressure was essentially built within the shell. Within the normal parameters of allowable tolerences rtc the gun itself played only an insignificant role in determining the pressure of a load. Most pressure, even so, are bored to minimum dimensions so any factor the gun dimensions play would be more apt to produce lower pressures than higher. Actual bore dia's can of course vary the pressure down the bbl, but this has virtually no affect on max chamber pressure.
This is of course not true in the case of rifles, where bore & chamber dimensions can play a most important role in developing pressure.
