Blunderbusses are large-caliber firearms with a short barrel that is usually flared. Lead shot is loaded through the muzzle and they are fired with a flintlock mechanism. The barrels were made of bronze or steel, and the purpose of the flared shape was not to increase the spread of the shot, but to make it easier to load the weapon. They were usually loaded with shot.
They were used from the 18th century onward and were not particularly accurate weapons for firing at a single target. It was for this reason that they were used in naval boarding and naval battles, since these were battles fought at close range. They were carried on small ships.
This particular one is also known as a blunderbuss. It was made in Barcelona and is mounted on an iron gun rest.