I can't directly answer your question, however I have an original Springfield Trapdoor that I have shot (about 10 years ago). I found that even with a barrel that had some pitting it was pretty darn accurate. Paul Matthews talks about being happy with a gun that shoots within about 1 minute of arc. Obviously he spent decades working on all of the variables.
I've done more reading than actual shooting and working up loads for my 45-70 Sharps, and what I have learned is that there are an enormous amount of variables. So, I wouldn't necessarily assume right away that it's your gun if your patterns are not good!
I'll let the experts give you a grouping number... my 45-70 groups at 100yds are nothing to speak of, even with a new sharps.