The 45-70 is a versitle cartridge but to shoot rabbits with it other then to see how much damage it does and not expect to get anything off of the animal, seems just foolish.... like shooting a rabbit with a 12ga Slug, in therory or course, but you see my point.
I am all for testing the range of a cartridge downloading it or loading it up, but these huge bullets are gonna shred up a bunny.
Why not pick up a 223 and load it down with BlueDot, I have and get great accuracy with a wide range of velocities from 22LR, 22Mag. 22Hornet, and full 223 power loads.
To each his own, best of luck to you in what ever path you choose.
-Aaron