54ball, I'm fairly certain that that rifle dates from the 1820s. It was bought new by my 4th Great Grandpa before he had any children, the oldest of which was born in 1827. The story was that he bought it while on his way from Montgomery County, NC to Pickens County, SC, where he was given a piece of land by his new father-in-law.
I'm also confident that it was, at one time a flintlock. My great, great uncle (Hovie), who is still alive and thriving, confirmed the story (without prompting) that the rifle was converted to percussion soon after or possibly during the civil war. Evidence of where the cock rested on the side plate is still evident. There is a slight deformation in the steel. The substantial corrosion occurred from use over the next several decades.
I imagine it wasn't cleaned, stuck in closet somewhere, and brought out occasionally. The humidity down here would make fast work of any neglected iron.
At any rate, thanks for all of the replies. I enjoy getting a lot of different perspectives.