I am presently playing with Savages and just reinstalled a barrel after having a muzzle brake put on.
This is how I did it and may not be perfect so approach with caution.
I chambered a FL sized case (used my die). Chambered the case and turned in the barrel until it 'hit' the case. Tighten nut while case and bolt is closed. That's it.
I test chambered other cases with proper chambering. Firing proved all was well. I neck size with a Lee collet die and have found no issues at all. In fact, the rifle shoots pretty well.
Here is a 'factory' link that has info about barrel install. Have fun with the Rem magnum..