The barrel looks like it's off a horse pistol. This is a British Dragoon pistol, called a horse pistol because they were carried by the horse, one on each side of the pommel of the saddle. Barrel length was pretty much a standard 12 inches. There were also sea service pistols that looked a lot like the Dragoon, but I don't think the barrel was as long. This one is American made.
Does your lock plate have any writing or figures on it at all. Look for a crown in the center of the plate, and possibly a date on the left end of the plate, or any other writing on it. These guns date from the 1700's up into the early 1800's. Pistol stocks under went drastic changes in style, but there was plenty of overlap.