Well, if we use the term 'gun' in its generic sense and include the way the rest of the world sees it, there is only one answer. The 'Kentucky' rifle.

http://www.allenmartinrifles.net/gallery-of-guns/herman-rupp-rifle-lehigh-valley/

If we use the term 'gun' in its literal sense, (don't ever say you are carrying a 'gun' to a Marine Drill Instructor, its a RIFLE) I would say that's pretty subjective.