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?

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?

This is, a 2P says, a great discussion.

JC

"...it is always advisable to perceive clearly our ignorance."ť Charles Darwin