So I made some mods and got my 30-06 box to work perfect with the 300 WM. Here is what I did. My stock is a Bell and Carlson Medalist.

I ground off the ears on the bottom of the mag box that held the follower spring from going out the bottom of the mag box. Ground them flush to the side wall of the mag box so the follower spring would pass the bottom of the mag box and rest on the bottom of the stock inlet. This lessens the tension on the follower spring. I used the flat spring follower, it works better in my opinion that the round coil spring follower.

Tweaked the mag lips to .510 or so using a DIY feedlip adjuster. Old screw driver cut off with a hacksaw and then made a hacksaw cut about halfway through about a 1/8" up from the end. You can buy these from Brownells, but I just made mine after seeing it.

That's it. With the reduced tension 3 rounds of 300 WM go in the box and cycle as they should, no issues.