Wall guns and punt guns fascinate me. Interesting carriage that one is mounted on, too. I've never seen one like that.
Some years ago a fellow had a contemporary wall gun for sale at a show I was attending. I was sorely tempted, but the $5,000. he was asking was a bit too much for my pocketbook.
There is an old story that I heard back in my native New Jersey about a wall gun. It seems that during the F&I War a series of blockhouses were built along the Delaware River, on the Jersey side, as a defense against Indian raids. A Jerseyman by the name of Holloway stood in one of those forts and used a wall gun to pick off an Indian that he had spotted standing on the Pennsylvania side of the river, somewhere around 1000 yards away.