|Springfield Model 1842. Photograph provided by "Older Firearms." Flickr Creative Commons (https://www.flickr.com/photos/35513864@N05/3292174456 : accessed 9 Jun 2016). Creative Commons Attribution-Share Alike License 2.0 Generic (CC BY-SA 2.0).|
I'm not exactly an expert on firearms, but my nephew knows much more about them. He served with the Army National Guard for two tours of duty in Iraq and is much more interested in them than I am. I put the "fourth cousins" in touch with one another in such a manner that I was copied on the messages.
After what seemed like a live Internet version of Antiques Roadshow, it was concluded that the gun was a Springfield Model 1842 and that it was one of the guns used in the war and issued with a modification that was present on the one in the family. When Union soldiers mustered out, they were given an option of purchasing their weapon. While we cannot guarantee this was the rifle issued to James M. Thornton, it seems likely that it was.