A "smooth rifle" by definition, is a smooth bore built as a rifle. Rear sight, cheekpiece, prehaps a patchbox and carving in the spots you would expect on a rifle. In other words ,any gun you would expect to be a rifle but happens to be a smoothbore.
Many were built as such for practical reasons. Some were bored out rifles. I suspect that set triggers would be a sign that the barrel was bored out.
In all things, there might be exceptions.