How about the 600 OK brass? what's the capacity on that?
I think the reason to go with the 50 would be the extreme BC's for the bullets. Just looking at ballistics graphs it seems the 30-378 will shoot almost as flat to 1000yds as the 50BMG. The better BC of the 50 makes it flatter past 1000 yds though. The 30-378 will fit an Enfield action comfortably and with a muzzle brake is rather pleasant to shoot in a 9lb rifle.