Well I figured id chime in and save you some time and frustration, with a 300 win mag mag box you are limited to 3.6oal with center feed, there are detachable mag assemblies out there that will accept 3.85oal with slight mods to the action and are a hassle. Also when purchasing a magnum bolthead be sure of the firing pin size, standard .473 or 30-06 boltfaces typically have the large pin as magnums usually don't, there is also a controlled round feed magnum boltface designed for the wsm's and rum's due to the large case and rebated rim but always check because the pins are only interchangable if you put a small pin in a large pin bolthead, but if you do its sloppy, I think you need a little more guidance, not insulting you, but maybe take your completely apart around someone who knows savages inside and out.