6" offhand would be just fine for a group with a DR. BUT - what is the load?
Is it a deer load only?
I've not seen ANY factory ammo that was worth a hoot as far as power is concerned.
There was an outfit in the USA who made some decent .45/70 ammo, but haven't heard of them lately.
IIRC they had a 400gr. solid and soft running 1,900fps or so.
My Pedersoli 1886/71 likes a hard cast 450gr.FP at 1,600fps. That's a decent moose/elk load (which I don't hunt any more LOL).