I wouldn't pay too much attention to Lindsey. He is probably referring to the "clock" style patchboxes. Yours is a variation on that theme, not common but not unknown either. I've had two of them at least. As to militia service... yes if he belonged to a rifle company though the fact that Mr. Sterns lived in NH leads me to think there is a chance he hunted with this rifle - or hunted and did his militia service with the same gun. At least there were big animals to hunt in NH in 1820. The conversion to percussion may suggest this also, though not very conclusively. Percussion ignition was well known in NE by 1825 and the requirement that each militiaman furnish his own arm wasn't suspended until 1832... (oops, in Mass. - I don't know when, or even if, this requirement was dropped in NH. It may have been in effect until the Civil War.)
Since you have a place and a full name, you may be able to run down some information on him. I'd start with the LDS Family history web site but you can also contact the HS in Amherst. For all we know, they have the muster rolls of the local militia companies. If he was a wealthy merchant he probably didn't hunt... if he was a farmer, quite possibly.
