You would have to trim the 300 brass as it is 2.62 long and the shoulder is farther forward. Better off with the 2.50 inch brass, less work.Yes, if you could find a BAR in .300 or .338WM, all you would need is a .416 Taylor chambered barrel. All of the case dimensions are the same. Even a BAR in 7mmRM would work I would think. I love shooting my Interarms Whitworth Taylor and the cases are so easily formed from .458WM brass. I haven't tried using .300 or .338 brass yet.