You can get 300wm mags in the standard 3.715, 3.775 or (I'm pretty sure) in cip length (3.85 iirc)
**EDIT my 3.775 300wm Accurate Mags measure 3.704 inside, I'm ASSuming that 3.715 mags would be roughly 0.060" shorter, so around 3.644. I suspect any factory loads will fit fine, it's just a...