AKwolverine,
Welcome. My pleasure. It is for my understanding too.
I never took the time previously to note how little size difference there is
between the unblocked-standard M70 mag box and the Wiebe XRM.
If the standard box is tight with 3 down, it will be tight for getting 4 down with the XRM.
Cartridges fatter in the shoulder area will be tighter.
The .375 H&H ought never be a problem.
Heck, the Pre-'64 M70 mag box holds 4 down with the factory standard .375 H&H sheet metal box.
