I'll add two things to this discussion:
(1) Slug your bore. Many Marlins, especially guide guns, won't shoot a plain base bullet worth a hoot unless it is .460". I own one of those. And you don't need gas checks.
(2) I think you'll get better terminal performance with a lead-tin bullet (no antimony) - something with a BHN of 15 or less. I know everyone likes those super hard bullets, but they are 1/2" hole punchers. Unless you are hunting armor-plated Grizzlies, a deforming bullet will work better. My opinion of course.
And a third thing for free - you can't flatten the trajectory of the 45-70, so no need to push them. 1400fps will kill an Elk just as dead as 1900fps.
Have fun!
