Frank,
What's the ctg. length capacity of that Mauser? Some of those longer, older BP ctgs are pretty long. Unless you plan to go with BP, they're actually counter productive. With smokeless, the 45-70 is case enough and easier to load accurate rounds than some of those.
You can load a 45-70, in that gun, to some pretty awesome power levels. If it was mine, I'd stay with the 45-70 and see what all I could do with it.
IMHO,
DF
