The first three images are examples of a Blunderbuss, a Musketoon, and a Boat Gun. Note the similarities as well as the difference. The blunderbuss in this example has a barrel with a diameter of 1.5 inches, the Musketoon has 1 inch barrel and the boat gun has a diameter of 1.25 inches. While the barrel of the Musketoon appears to flare, it really is just shapped like a cannon. The pintle mount of the boat gun give the weapon added stability during firing as the recoil of the weapon is absorbed by the ship. The boat gun in effect is a small swivel gun.
Blunderbuss, 1710
Musketoon, 1757
Boat Gun 1700s era
17th Century Wheellock Blunderbuss
18th Century Blunderbuss Pistol from England
18th Century German Blunderbuss
18th Century French Blunderbuss
18th Century Spanish Blunderbuss
18th Century 29 inch, English Coach Gun (Blunderbuss)