The .45ACP headspaces on the case mouth, except when excessive headspace calls on the extractor to keep things under control. In regular trim, (17,000psi max) excess headspace is no big deal.
Most chambers are long, most brass is short, the average ends up with most guns having .015" to .020" excess. I've seen loose military 1911's with .030", and still working fine. In standard loads, no problem. In the 40K loads of the 460, problem.
The leade is the short rifling-less section and the taper of the beginning of the rifling. If it is short, or the taper to the rifling is steep, pressures will spike. Again, in standard .45ACP load at 17K, no problem. In the 40k 460 Rowland, big problem.
All calibers have the dimensions specified, and manufacturers strive to stay within the specs. Some specs vary, as in the differences between civilian .223 leade and military 5.56. The military leade is longer, more tapered and keeps the pressure spike down. If you shoot normal-pressure military ammo in a civilian .223 chamber, you can have over-pressure problems, even though the ammo is OK in military chambers.
Tell us what happens, and how the new barrel looks.