Good questions, JayCee!! Very thoughtful.
Rocketman, wouldn't it be that considering that lateral support will not be the same in a larger barrel, the wad will expand more in a "conic" way than in a "cylindrical" one (as it would in the same gauge barrel) and this would account for less wad base skirt making contact with the barrel and thusly less friction? Yes, but at, I think, a trivial level. The bulk of the friction will come from the shot cup, I think. The acceleraation will cause considerable pressure within the shot column; which will be felt also laterally by the shot cup petals pressing against the barrel walls. However, if the wad was not so slick (low coefficient of friction against the steel), the friction would cause excessive wad melting. We don't see that.
Could it also be, as it was mentioned, that a slight increase in internal volume will procure more oxygen that will help the powder to burn more efficiently? No, the powder is a propellant and contains its own sufficient fuel and oxygen supplies. A fuel requires oxygen from the air. A propellant will burn equally well in a vacuum.
This is, a 2P says, a great discussion.
JC
