Thanks for posting those pics Feltwad. I like the one in the top picture, very early, maybe 1770's or 60's. Have to see the lock better to know for sure.
There were alot of large shoulder mounted cap lock guns used here in the Midwest. Usually 8 to 4 bore with 38" to 48" barrels. Mostly single barrels , but a few doubles. They are usually English or Belgian, none were made here.They show up around here all the time, usually in "well used" condition. This area was a major flyway with all it's rivers. They are universally called "Market Guns" in this area.
Punt guns were also used here. They very rarely show up, probably as they were outlawed after time and seized by authorities. They seem to be "home stocked" and are exceptionally crude.
Here's my version of a Hudson River "Bank Gun".