Technically, a
rifle is a
firearm that has a
barrel length of
18 inches or more and whose
bore has
rifling grooves.
A long arm with a barrel length below 18 inches is considered a short barreled rifle or SBR and is considered to be an entirely different animal in the eyes of the law, it is categorized Class III, along with shorty shotguns and burst capable firearms.
A shotgun with a rifled barrel is still considered a shotgun since it only fires shotshells and/or slugs.