Bob,

I believe the rule of thumb is to use a bullet of bore diameter, that is, the diameter across the lands.

It should be somewhere near .450" for a .458. Paul Matthews goes to some length discussing the proper bullet diameter.

There are other complications also. For instance, most A. Henry rifles have tapered bores and grooves. That can make a big difference. The bullet cast from the original mould for my A. Henry .577BPE has a tapered shank and is made to be paper patched. You just have to wade into the water and see how it goes.

Best,
Curl