It looks like it had an iron sling swivel, which would make it military. Probably off a flint lock. although the 1841 Mississippi Rifle was a cap lock. The French Charleville musket had the sling attachment in front of the trigger guard, so that gun is ruled out. Here is what I found.
The above photo is the 1841 Mississippi Rifle. It's not a match.
Next is the British Brown Bess, again not a match.
Last is the 1816 Harpers Ferry musket, and it sure looks pretty close to me. The screw hole in the rear part of the trigger guard is in the right place, along with the sling swivel. I'd say that's a great find. Did you find anything else worth telling about?
It is definitely a civil war era British Enfield .577-caliber Rifle Model P-1853's brass trigger guard, with it's long narrow tail broken off at a screw-hole. Search that area carefully, because finding that large piece of brass indicates there are very probably some more parts of that Enfield in the nearby vicinity, along with other civil war military relics. The Enfield's lockplate and its parts, and the barrel-bands which held the barrel to the wooden stock are iron, so you'll need to dig some iron signals to find them, if they're there.
To help you recognize the Enfield gunpart which held the iron trigger, one of the photos below shows it. The other three show what a less-damaged example of your find looks like.
By the way... BosnMate is right, this model of brass Enfield trigger guard did have an iron loop swivel for the rifle's shoulder-sling, but it's mostly rusted away on your find.