As I noted earlier, the muzzle velocity is a product of the pressure - displacement curve times wad base. Higher muzzle velocity from a larger bore barrel using a chamber reducer simply means that the wad base area increased more than the pressure decreased. That is, the powder charge was more efficient in the larger bore. This is, to me , slightly counter-intuitive, but not at all unbelievable. If solid data says it is so, then I accept that it is so.

I doubt that barrel friction enters into this issue. I showed not too long ago that the expected frictional forces are trivial when compared to the wad base forces.