Yep - If he bought it in the -80's, it musta been the 1880's.
It looks like what used to be referred to in Ned Robert's book
The Muzzle-loading Cap Lock Rifle as a 40 Rod gun, from the late 1800's.
I would wager, if you tried it, the round part near the muzzle is actually a removeable false muzzle, aligned via pins, which was used to guide the bullet during loading, then removed for the shot.
If the face of the muzzle just has the pin holes (usually 4) in it, then the false muzzle which matches the round part of the barrel has gone missing.
Since you picked it up for a song, start singing - and you better dance your happy dance.
You made a score worth $1500-$2K, minimum.
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