Roy I don't really know....never measured the capacity of the 300 WM necked down. The Mashburn has slightly less taper. Six of one/half dozen of the other I think.
I think a lot depends where you seat bullets but we can bet any difference is pretty fractional. Whether he realized it or not,what Jarrett did is reincarnate the Mashurn in a slightly different form. But that's true of a few other 7mm magnums as well.
The 300/7mm RemMag has almost no taper, and the longer body and shorter neck so a bit more powder capacity. The plain 300WM/7mm is also reffered to as the 7mm Practical.
My 7mm Mashburn marked custom rifle is, I learned when starting to shoot it and inspecting the cases, actually a 7mm Practical. I can run a 300WM case into a 7mm neck sizing dies and have perfect brass. That is one of the things about buying someone else's already built custom, no way to really know the chamber dimensions until shooting it and measureing the fired cases or doing a chamber cast. I am OK with my slightly larger chamber and easier to form cases, and the beautiful engraving of the 7mm Mashburn is so pretty (on a tapered octagon barrel) I am going to leave it as it.
