I don't pay a lot of attention to weather the guns are perfectly time period correct as they are close enough for me. I really enjoy the show and hope it continues for a few more seasons.
I am not educated enough about post Civil War arms to notice the difference as are most people. The Henrys looked good. They are always portrayed to be more powerful than they actually were. ( I think). And the Sharp's mercy shot was a little over the top (I'm sure it was possible but can't really buy a man just grabbing an unfamilier weapon and making a perfect 1st shot) but was still entertaining.
The historical details have never been too important to Hollywood. I think they get pretty close and tell a good story.