Figured the first poster has enough on his plate..throat, ball seat, lead (sometimes leade in older texts)...are several parts to the secion ahead of the chamber but before the rifling actually starts.
Can sometimes get away with a slightly undersized soft lead bullet using black powder...sometimes have no real choice as the chambers can be cut too tightly to allow a full diameter bullet to escape the case mouth...but that's not somthing you find on many modern rifles (at least not reputable makers). Even when you do find that problem, unless is it a five figure collectors item, would tend to have the chamber adjusted rather than living with that tight neck.
Basic rule of thumb test (not an exacting test) is to try various diameter cast bullets in a fired.unsized case neck...looking for the largest size that is a slip fit into the case. That case has contracted a bit after being fired, so whatever size that will barely fit into it should have enough room when the case is fired to cleanly release the bullet. Will find many 30's will do their best mid-range work with .310' bullets (and some with .311's).
Poster's "problem" is that there are 3 30cal. rifles on his list, and it would be unusual for all three to have the same dimetions.