.. Army Basic Training, the drill sergeants told us that "guns" were on battleships, those things we were carrying around were "rifles."
And in the NAVY, those same battleship weapons are called "rifles" !!!!
16" 50 caliber
rifles. The Navy also uses the alternate meaning of "caliber", which expressed the length of the gun barrel in terms of bore diameter. A 50 caliber naval rifle has a barrel 50 x bore diameter in length.
So a 16" 50 caliber rifle has a barrel 50x16" long (which by my math works out to be 65 feet).
The Military uses a very specialized terminology, intended to, but not always effective at, reducing confusion about what item is being discussed.
LAWS use a different set of terms, with the definitions being defined in the law.
Regular conversation uses terms in what is usually the sloppiest manner possible, and relies heavily on context to get the intended meaning across.
The Army has its own ideas about what is, and isn't a "gun". The M16 (or any infantry small arm) isn't a "gun". To the Army, a "gun" is a cannon, and one that is intended primarily for direct (line of sight) fire. A Howitzer is a cannon intended (primarily) for indirect fire. In WWII we had both a 155mm M1 Gun, and a 155mm M1 Howitzer, and they were NOT the same thing.
Bottom line is, in the military, no matter what it really is, is what the guy who outranks you says it is, until someone who outranks him, says differently.