From tromix:
I spent some time today with your upper and your ammo.
I rechambered your barrel with the current spec reamer which has .030" additional Lead/Freebore.
The area directly in front of the chamber with no rifling is the Lead/Freebore. This distance on the 458 SOCOM is .080" (U-V on the print).
After the .080" Lead, starts the Throat which is the tapered section where you see the rifling go from nothing, tapering up to full rifling.
This distance is .267" (W-V on the print) The diameter of the Lead is .459" so you have a bit of a press fit with your bullet sized to .459" as it enters the Lead and you have no room for the full bullet diameter once you go past .080" and enter the Throat.
I also chucked up one of your dummy rounds that looked like it hit the rifling on one side, and I was able to get the case body run-out just behind the shoulder down to a ¼ of a thousanth (.00025) I was surprised it was that perfectly round. Then I moved the indicator over to the bullet and found there was .005" runout there. So your bullet either is not round, or it is not seated straight, or both.
I also pulled the SAAMI print for the 45-70 to see what kind of Lead/Throat it is running. Oddly, it has zero Lead, and only .091 Throat.
Then I pulled the SAAMI print for a more modern cartridge, the 6mmARC and found that it has .090" Lead and a .220" Throat. Keep in mind, that cartridge was designed specifically to dangle long ass bullets way off the front end of the case.
Anyway, I cycled your dummy rounds through your barrel and they seem to chamber and eject without hanging up, but the bullet is up against the lands with the current seating depth.