I would think it's "sights" or optics that limit the range. If you can see something well enough at 200 yards with open sights you can shoot it with a 45-70 with open sights; however, if you can see it well enough at 300+ with open sights you can shoot it with a 45-70. If you want to scope your 45-70 it becomes a long range gun; although I think long range is a subjective term.
