The wall guns of the flint and percussion era might have given rise to the idea of punt guns.
They were of large bore, and were mounted on pintle on the fort walls. Too, they would shoot fairly well due to the large bore sizes, rifled or not.

Reminds me of a 4 bore underhammer 'gun' my brother built for a fellow who wanted a large gun, but bigger than the Billinghurst & Verner underhammer match rifles in Robert's book. I wanted T. to line it with a large bore slug barrel, .50 or .60 cal for shooting huge elongated bullets, but that didn't happen. The fellow who ordered it did not want the added price.

As it was, we made a couple moulds out of hard-rock maple to cast 1" balls of pure lead - around 1,750gr. The barrel was 2 1/2" across the flats, some 40" long and the gun weighed 52 pounds as I recall.

With tang mounted aperture sight and post front, along with the 12 dram charge and patched round ball, we were able to shoot very close to a 3 foot group on the 300 (+10m from target stand to berm) meter berm. It certainly would do for a man on a horse at that range. I recall a fairly large amount of dirt and rocks moved at the embankment, at each shot. The gun (& your shoulder behind it) would move backwards some 12inches at the shot. It could not be stopped, and simply slid backwards. You could not say it kicked, but just moved on the bags, with or without you behind it, perhaps a bit more on it's own with just a hand on the butt. Mid 80's iirc.