The difference between them is pretty wide and they were not used in the same fashion. The Nock gun is a volley gun, which shoots a rifle or round ball. Basically it's musket with multiple barrels that each fire one round. This was meant to be used high in the rigging of the ship to shoot down on groups of enemy sailors. This is a "long range" weapon. A blunderbuss used buckshot, bird shot or really anything that you could fit in the barrel. It's an early predecessor to a shotgun. Due to it's short barrel length, ammunition and design, it was a short range weapon, but this is what made it effective for ship combat. But the Nock gun wasn't in use for very long because it was difficult to reload and the recoil was very painful and caused injuries.