The odvious difference is the relativce case sizes. The other less odvious difference is the pressure levels that each are loaded.
That means the .45-70 has to burn more powder to get the same velocity levels as the .460 with equal weight bullets. That means it will kick harder.
Can your buddy get the ballistics he needs with the .460 loaded at much higher pressures or does he need what the .45-70 can give him ? Loaded to nearly the same pressure levels as the .460, the .45-70 gets very close to the .458 Winchester in performance. E
