Ezra, "Wall Guns" were used to defend things like castles. They often had very long barrels, were "at least" a one-inch bore, and also had a "yoke" at the balance point. With the long barrels, they were very accurate.
That yoke was placed in holes in the ramparts, such that the wall would take the recoil impact, instead of a man's shoulder.
George Washington had several, and it was demonstrated that they could hit a target at 600 yards - not bad for the 14th-16th centuries!
I really think that you should build one, then demonstrate it for the rest of us.
Someone who more than likely knows more than I would be Richard, aka Pukka Bundook on the forum here. Drop him a message.
Hope that this helped you a bit.