Stagger feed mag opening is not compatible with the Accurate Mag bottom metal. You'll have to mill the magazine opening to match a center feed magazine opening. This is why I feel a regular centerfeed magnum action is a better candidate for a RUM repeater build than the old RUM actions.
I'd use a 300wm magazine, but I think either will work. However, either one will have to get the front rib milled out. The 300win and the Lapua have a shorter shoulder than the RUM.
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