A .458" bullet will be the easiest to source of course and if it chambers you should be fine with that.
A Henry falling block I have right here in hand has a bore of .449"
and the lands come right to the case mouth from the looks of it.
I doubt this one would chamber a .458" bullet so Mike's advice would be worth heeding for this rifle. Your's may differ.
You mention a "bore" of .458" tapering to .455 but I'm sure you mean the groove measurement, right?
When using black powder, I only focus on the bore diameter and think the groove depth is rather moot.
Are you planning on using real black powder or smokeless?
