It is definitely a brass barrel-band. Specifically, it is from a US Model-1841 "Mississippi" Rifle. (Important note: The Confederates manufactured some copies of it.) I still own an exact-duplicate Reproduction of a US Model-1841 Mississippi from my years as a civil war battle re-enactor, so I checked your barrel-band with mine's, for confirmation of the ID that I've given you. To learn more, and see photos (in a slideshow), go here:
The National Firearms Museum: U.S. Harpers Ferry Model 1841 Mississippi Rifle
And for "mouse over to view" enlargement, showing your barrel-band close up, go here:
http://shop.cohenantiques.com/firea...-1853-mississippi-rifle-with-peace-flask.html