Sylvester Roper was a great example of the classic American inventor – he had a wide range of interests, and impacted technological development in more than one industry. This shotgun is a design he patented in 1866, which uses a 4-round magazine of the 12ga or 16ga shells (in its earliest versions, it used percussion-fired reusable chambers like an early Gatling gun). It has an unusual mechanism which fires from an open bolt. This particular example is missing its magazine internals and has a broken bolt, but is the first actual Roper I have come across.
Roper went on to work with Christopher Spencer developing the pump-action shotgun, although he is best known for his work on motorcycles (which he was actually the first inventor of, according to some).