What separates a fowler from a musket is a matter of a few facts. 1. No provision for a bayonet on a fowler and 2. leaner and lighter carbine sized muskets, 3. Most fowlers were smoothbore, Ive read on occasion that some were rifled like todays modern day reproductions, the procedures for rifling a gun were long and complicated in the 18th century which is why most rifles were made in an octagon shape or pentagon shape, they could be gripped fairly easily for cutting groves. Rifling a round barrel was not impossible just not practical, I tend to think most fowlers would have been rifled as a sort of customiced upgrade, if the barrel permitted for it, early rifling was cut deep. Ferguson rifles were one round barrel that was rifled however military contractors not civilian 4. Ive seen fowlers with full round barrels and half round half octagon, typically having flats at the breech end provides for a more stable surface for sighting. 5. Some Hudson Valley Fowlers were very large, and had bores up to .80, I tend to refer to these and buck and ball guns.