Bidyanus,
Getting back to your barrel measurements, there is a good chance that the bore and groove is tapered, with a larger diameter at the breech compared to the muzzle.
Apparently some BP doubles were built with the taper from new, in an effort to reduce problems with fouling.

This is described in Graeme Wrights book in the black powder section.

This means that if you measured .470" and .472" at the breech end of the barrels, they could measure a fair bit tighter at the muzzles.

Graeme also points out that some of the rifling types from that era can make it difficult to obtain accurate bore and groove measurement.

If you don't already have a copy of "Shooting The British Double Rifle" by Graeme, now in it's 3rd edition, I would suggest you get hold of a copy.
There is a hell of a lot of valuable information in that book, and it is all based on many years of genuine double rifle experience.
He also describes how he has dealt with accuracy problems in BP rifles similar to the situation you have with yours.

It will be interesting to hear about the dimensions of your barrels when your friend gets a chance to check them for you.