Standard chamber pressure for the .500 3" Nitro Express is 40,610 PSI. The old technology standard was 16 tons bolt thrust (which isn't chamber pressure). The old 3 1/4" version cut roughly a half ton off the old 3" standard, but that isn't PSI. I can't see any sense at all in chambering a current gun for 3 1/4". I wouldn't want a conversion on a shotgun action in either caliber.