I have to respectfully disagree with the above, at least in part. The dimension that really matters is the throat diameter, not the bore size. This is well known among cast bullet shooters and is noted in many CB loading manuals.
Relating directly to experience with the .45-70, I have a Petersoli rolling block with a beautuful octagonal barrel chambered to .45-70. Nice tight chamber, flawless lapped bore - but the later measures just .454"! The throat is .4575". I size the RCBS 400FNGC to .457" and the rifle shoots moa with dual aperture sights. It even won a few CBA matches when I shot it competitively.
.