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.