Kampervan,

"Punt guns were used in the 19th century to hunt waterfowl. It's still possible to pick them up second hand."

Used to be used here on the Chesapeake Bay- they are illegal here now, and considered WMD ( anything over a specified diameter. ) I think the 26mm flare pistols are about the largest diameter legal firearms. Some muzzle-loading cannon are available, but require licensing.