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.
