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I would start with a cast bullet in the 405 to 430 grain range at a velocity of 1300 to 1500 FPS. You do not need velocity in the 45-70...it just a waste of powder and an increase in pain (unless you are on some EGO trip).
I use Unique, 2400, and Blue Dot with Oregon Trail's Lasercast 350 and 405 gr. cast bullets. Good for shooting steel out to 800 yards (the limit of my experience), some friends shoot long range with the 45-70 but I have not tried that....yet.
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I am not into nose bleed recoil!
The velocities you said are what I am shooting for. I am looking for accuracy and killing hogs mostly within 100yds. Ive heard that a big cast bullet doesn't let them run too far.
