[mlangsdorf]

Single shot, breechloading "Army Rifle" is probably a Springfield M1873 (HT110 or Adventure Guns p 30).

I don't know the Boot Hill time scale, but the other plausible candidates are the Sharp M1859 (Adventure Guns p 26) or the Springfield M1861 (HT108-109 or Adventure Guns p 27 but it's a muzzle-loader).