My rcbs 325 (non-gas check) works well with bp for 3-4 shots, which should be sufficient for your proposed purpose...however, I do like the Lee 405 HB bullet as it carries a lot of lube. I've been playing with these in a Uberti 76, 45-60. I'm getting 1085 fps with the Lee HB, but with your 45-70 case length, I guess you'd be closer to 1300 fps.

A good mod for the Lee HB would be to mill the top mold surface to make a larger meplate.

With smokeless, I've pushed the rcbs to about 1500 fps.

Hope you get that barr,

Slim