Borden811 et. al.
Though I have cast my vote for the Whelen in this fun discussion, like I said, there's really no wrong answer. So, in defense of the "hard to find brass" argument against the 350 RM, can't you just make brass out of any belted case, just by running it into the die and then cutting to length (maybe not necessarily in that order). I would think .338 WM would be good to start. I reckon you might have to turn the necks afterward if they got too thick, but otherwise I can't see why brass wouldn't be easy to make.
Cheers,
Rex
