Roy, it seems that modern plastic wads will, indeed, make a good seal in a larger bore than might seem reasonable. The reduced pressure is pushing against a larger wad base. Payload acceleration, which determines muzzle velocity, is controlled by force on the wad base. This force is the pressure times the wad base area; smaller pressure times larger area equals similar accelerations. I am yet to see conclusive data on comparisons of velocities, though. Market acceptance of chamber reducers would indicate reasonable performance.