On my Rem 700, I'm running a 3.850" Wyatts box, my COAL is running 3.720"- 3.850", depending on bullet I'm using. The biggest Wyatts box is what you want. It is not a modified Lapua box, it has the same width as for all belted mags and feeds perfect as is.
Cheers.
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I have been wanting to build a .338 edge, but I don't know what bottom metal to use. Does it need to be a the bigger bottom metal with .338 lapua mags or do you use a standard M5 bottom metal with mags for like a .300 win mag?