For 10ga you can use our 1 3/4 oz slug loads in 3.5" plastic cases.
We get 1800 plus. That is 765 gr slug. We use 110 gr of
IMR 4227 powder. Gun is weighted.
The SB2 rifle and SB2 shotgun frames are only different in firing pins.
Both are alloy and heat treated. What I'd do with the 45-70
for fun is lengthen chamber for 45-120 or make a 450 NE.
Others get an extra barrel fitted for varmint calibers.
Here is picture of 12ga turned solid brass slugs we tested.Ed