I'm rechambering my Lyman 1878 (Pedersoli Sharps 1877 copy) from 45-70 to 450 Nitro-Express. I didn't consider the 45-100, 110, 120, etc because I love British big game cartridges and always wanted a 450NE. Both the 45-70 and 450NE are .458.
The Lyman has a forged receiver and a cut rifled barrel and I think is one of the best looking rifles I have. The quality and workmanship surprised me as I didn't think an Italian clone was this good. Making this a 450NE just adds to the exotic appeal to for me.
Ammunition, Brass and dies are readily available, if not the least expensive in the world. Chamber reamers can also be had with no problem. My gunsmith is on board with this and I'll be dropping it off in a couple of weeks.
I opened a ticket with Lyman and they told me to use the 45-70 loading data for the lever action Marlin 1895 and Winchester 1886. The max pressure for those are 43,000 cup. The 450NE's is at 39,000 cup. So all things are good for handling the extra boom.
I am a little concerned about the stock holding up and I'm likely to do a really good bedding job to help it out as much as I can. Maybe an Acraglas with atomized aluminum (aluminum powder) mix.
Anyway, back to velocity. How fast can I push a cast .458 cast bullet? As I said, the barrel is cut rifled and is slick as snot. It never leads up on me and is crazy accurate.