It's just a term. But it IS accurate. In the golden age of flintlocks, smoothbore rifles were made exactly like rifled rifles. They had the same stock design, the same sights
I do agree that "smoothbore rifle" is just a term, a convenient and popular way of describing this type of firearm.
I wonder, though, about the accuracy of the term. What makes a rifle different than a shotgun? What makes one a rifle and the other not?
Is it the name that people use that determines what something IS or is it something else? The fact that the term has been long used and by knowledgeable folk does not mean that it is accurate, only that it is convenient.
Making an inference here.......you are saying that it is stock design and sights which determine that a firearm is a rifle. Is that correct? (I don't want to put words in your mouth if that is not what you mean.)
The Italian arms company, Pedersoli, manufactures a firearm named the Kodiak Express Double Rifle. One version is in .72 cal, rifled barrels. They also make (or made) a 12 gauge SXS shotgun that uses exactly the same stock. Would adding a rear sight make it a rifle?
Does bore size make a difference? Smooth rifles tend to be in calibers much smaller than the .72/12 gauge of the Pedersoli. Green Mt. has a smooth rifle barrel for sale in .40 cal. Most that I have seen are in calibers more traditionally found in rifled barrels. Until recently, I had a smoothbore .54 cal barrel for my GPR. Was that a smooth rifle? I thought of it as a fowler of sorts.
Pete