Joe, in my book "Gun Craft" there is a chapter specifically on this topic -- "Jointing and the Circle." (Pugwash is correct in his comments about rejointing pushing contact off the circle on traditionally made guns.) I worked with 5 of Britain's best actioners to write it, and it has diagrams drawn by gunmaker (actioner) Bob Turner with the dimensional tolerances for SxS shotguns and double rifles. It also has photos of the replaceable hardened insert that some gunmakers now use to keep bearing on the circle if a gun is rejointed.
As Stan notes it is adapted from an article I wrote for SSM. If you can't locate a copy, I'd be happy to e-mail you a copy of the chapter. PM me with your address, please.