So, a derringer is a small pistol, usually with two shots, made for close-range self-defense. Normally they use, well, pistol rounds, like a 9mm, which has like, 700 joules of energy or someshit like that. When you fire a lightweight gun, you definitely get some kickback from it, even with a pistol round.
A 45-70 is a big-ass rifle round with something like 4000 joules of energy behind it. You uh, you put that in a little derringer and pull the trigger, both you and your target are gonna feel it.