30" would be VERY long for a .22 rifle...at least at modern standards. I'm not familiar with black powder rifles in a caliber that small.
Hmm, octogon barrel, no indication of a black powder breech...Going to have to do some research.
ETA: Do you see any indication of a dovetail for a front site? I did find an 1885 High Wall image, in .22, with a 30" barrel. The barrel profile looks like it COULD potentially be a match, given the screw in breech end of your barrel.