Civilian sporting arm.
Drilling or three barrel gun. It appears there is a British Arrow on the breech. The stock though is distinctly American. Crescent shaped buttplate with American Eagle inlay shows its American heritage. It's unusual as the rifle barrel is atop of the two smoothbore barrels. Generally, in a German drilling, it's reverse with the two smoothbore atop the rifle barrel. That's another hint that suggests to me that is American. Try to see if there's markings on the barrels.