Looked at 50 B&M load data (a very interesting cartridge) almost exclusively imr4198. The B&M 50 uses rum cases keeping the slight taper but also headspaces off the mouth assisted by exclusively chambering in cfr actions. Might turn me to a mauser guy yet.
Guess I'll be off to town tomorrow after a couple pounds of 4198.
More on 50 B&M
http://m.b-mriflesandcartridges.com/50-B-M.html