Pooch, the Rule of 96 is a good guideline, but it didn't come down from Sinai written on stone tablets. Example: Per the Rule of 96, you shouldn't shoot a 1 1/4 oz load in a gun weighing less than 7 1/2 lbs. I've shot 1 1/4 oz hunting loads in guns at least a pound lighter than that, and while I would not want to use such loads for volume shooting (targets, doves, driven shooting), they're OK if you're hunting pheasants, the limit is only 3 a day--and, of course, assuming that the pressure is appropriate to the gun in question, which is the key issue. (From a comfort standpoint, recoil is also important.)
Greener came up with the Rule of 96 back in the days when, at least in Great Britain, all loads containing the same shot charge were more or less equal. That's why British guns, for many years, had a specific shot charge stamped on the barrel flats as one of the proofmarks. Well, my vintage Lancaster is marked 1 1/8, but were the chambers on that gun to be lengthened to 2 3/4" and were it to pass reproof, I still would not want to shoot most American factory 1 1/8 oz loads in that gun. Most of them exceed the pressures to which vintage British 1 1/8 oz shells were loaded, and they may well exceed even current British pressure standards, SAAMI service pressure being somewhat higher than standard British (CIP) service pressure.
