Wow, what a beautiful rifle!
Judging from the depth and configuration of the grooves, I am pretty sure it would need a patched round ball. It would be good to know the rate of twist, but I understand that a lot of the Continental rifles had faster twist rates than American rifles of the period. If this is the case with your rifle, modest powder charges may be indicated rather than heavy loads.
has a good eye. As he pointed out in post #4, the barrel may have been lined. If the entire bore looks as good as the muzzle, it ought to be a good shooter. Lining the barrel would decrease the collector value, but if you want to shoot it, lining the barrel may be the best way to restore a rifle to shooting condition.
I would very respectfully disagree with the suggestion that it started as a Hawken or "plains" rifle. In using the scrolled trigger guard finial on their rifles, the Hawken brothers were following a trend rather than setting one. You see trigger guards similar to this one on a lot of old English and German rifles of the mid-19th century. The "shotgun" buttplate on the subject rifle, as well as its overall architecture would be atypical of American plains rifles. I don't know of any original Hawken rifles that used back-action locks, although these were very popular with some builders of percussion rifles, such as the Wurffleins of Philadelphia.
A lot of the German hunting rifles you see are very ornate, with extensive carving and engraving. This one is very plain, in comparison. It looks like a rifle of excellent quality but built for utilitarian purposes.
Thank you for showing it!