Re: New Build .300 Win Mag Question on cartridge length
It is your choice. Depends on if you want to touch the rifling, if possible, or get need to get the bullet closer to the rifling. I'd say load a dummy round and do some measurements. Maybe work up some loads and see if it will shoot.
Wyatt's mag box does require some minor machining. Have done it with a dremel tool in two SA rem 700s but it was slow going. I'd also say that you could have issues with feeding, might need a different follower or perhaps some ramp work if you seat bullets longer, you never know.
I just suggested you try the factory magazine arrangement with a hybrid and if it doesn't work to your satisfaction get the Wyatt's mag box. I have one on my post 64 model 70 for a 300 RUM when I used to shoot accubonds. It feeds just fine with original follower. Can't say the same for my 6mm-284 in a SA Rem 700, need to do some changing to make it perfect.