SH, thanks for jumping in with that information. Fortunately for shotgunners, the rifle analogies don't hold up. Shot is at least semi-fluid and plastic wads are very compliant. Ergo, they will fill a larger bore without large complaint. I think you were looking at a slight shift in favor of increased wad base force via enlarged wad base area vs lower pressure; the wad base area increased more than the pressure dropped. And, the powder found the larger bore burning conditions favorable/efficient. As for drag, HMWPE (wad material) has a sliding coefficient of friction on steel of about 0.02, as I recall, so it would take a very large change in lateral pressure on the wad base skirt and/or on the shot cup to have a significant effect on drag. The wad base skirts are going to see very similar internal lateral pressure in both barrels. The shot cup bearing length will shorten up by the volume ratio, say 6%. That just isn't going to account for 1200 fps to 1300 fps; a 17% increase in energy.
