This is probably a dumb question that I have simply not thought through long enough to figure out the answer for myself, but here goes.
A standard shell, say a 12 gauge, still has a standard shell diameter today, and a 12 bore barrel at one time had a defined standard inside diameter, and chokes were measured in terms of constriction from the bore diameter, which they still are today.
But the inside diameters of barrels from different manufacturers today vary a fair amount for any given standard gauge, plus some manufactures additionally overbore their barrels to, in theory, reduce recoil by reducing velocity. So I guess you might conceivably end up shooting a 12 gauge shell in a 13+ bore barrel. Yet it’s choke is still determined from constriction to this oversized-for-gauge barrel.
Doesn’t this make the definition and measurement of choke as the constriction from the actual barrel I.D. now fairly meaningless? Wouldn’t it make more sense now to re-define the choke size on any 12 gauge barrel as the inside diameter of the last 1 ½ - 2 “ of barrel, and wouldn’t this be a better predictor of what to expect in pattern? It seems hard to believe that the same amount of diameter reduction on two widely differing barrel diameters would both meet the practical desire of specified choke size which is percentage of pellets within the measuring circle for that shot load.
