Semisane is right about the new Great Plains having a 1-60 twist. Although the older ones did have a 1-66 twist and then in the 80's changed for some stupid reason. Then old rifles were and still are a very accurate roundball rifle.

With that 29" barrel I am starting to think it might be a very old Trade Rifle. I am not certain but I thought the very early trade rifles had double triggers then they changed over to the single hunting trigger. The Trade Rifle is 1-48 twist and will shoots lots of stuff. I have a Trade Rifle in Flintlock that I shoot all the time. An excellent rifle.