The .45/70 Derringer is a joke, imho. There is no way to get a decent second shot. Takes 2 hands to relocate it in your hand and cock it for the second shot.
Has anyone been able to chronograph one? I've not heard of that.
I was able to 'make' the 7 yard, 12 shots in 10 seconds with my 4" .44 Mag. with full power loads using 275gr. SWC's @ just over 1,210fps - all 10's, BTW on a B27 target.

Much better in my opinion than a 2shot derringer.

The only ctg. Derringer I have fired was a .45 Colt with full power 250gr. driven by 6.6gr. (I think was the load) of bullseye. They would likely make 500, maybe 600fps at the most, I expect from the 1" of actual bl. ahead of the ctg. mouth. The recoil stood the little gun straight up in my hand. It then takes 2 hands to get it back into your grip and cock it - oops - too late, bear's gotcha.