Hi Adrie,
It is not that they are special but that they served purposes different from rifles and tended to be more general purpose. However, there is a lot of variation in fowlers, which are smooth bored, depending on their purpose. The typical upland bird hunting gun or fowler from Britain was very light and well balanced, and made shooting birds on the fly possible. In contrast, the big waterfowling guns are usually heavy with very long barrels meant to shoot at ducks and geese on the water from longer ranges. The big barrels handled heavy charges of powder and shot capable of being lethal over a long distance. A good upland bird gun will fit the shooter such that the sighting plane is instantly aligned with the eyes when the gun is raised to the shoulder. It is balanced so it can swing to follow a bird in flight and fires a lethal pattern of shot out to 25-35 yards. They also will shoot a single large round ball (patched or unpatched) with accuracy sufficient to reliably kill a deer out to 50-60 yards (or meters). As a result, they are a good general purpose gun compared with rifles. During the 18th century, the vast majority of civilian guns in America and Europe were fowlers or quasi-military muskets, both of which were usually smooth bored.
dave
