The accounts of the Rappahannock wallguns were they could hit a sheet of paper at 600 yards. The German mercenaries during the Rev War used them against us. British used them too, but mostly with artillery.
I made one with an Ed Rayl barrel. Barrel was 54 inches long , 1" diameter bore, rifled. I welded a hook to the bottom of the barrel that protruded thru the stock. This was a common Dutch/German trait. I could hook it over whatever was available, branches, tree trunk, gunnels on a ship. Or drive it into the soft earthworks of entrenchments. Last time I used it I shot a ball and I think 350 grains of 2f. At reenactments the charge was even higher. I got out of reenacting probably 15 years ago. My father has it as he still goes to events in occasion.