Ah, Lynn, quite an opening barrage from I-don't-know-where, but since you have zeroed in, I'll be glad to lob some counter-battery fire in your direction!
Weapon: "An instrument of ANY kind used in warfare or combat to attack and overcome an enemy". The Shorter Oxford Dictionary, V2, page 2519. I think that might include just about everything, ...... artillery, small arms, tanks, swords, trench shovels, knuckle dusters .....
"Gun: A Gun is a piece of ordnance designed with a view to the tangent elevation required for any range being as low as possible. In comparison with a Howitzer of equal calibre, it is a long weapon with a high muzzle velocity".
"RBL (Rifled Breach Loader): The first RIFLED GUNS introduced into the Service (ie British Royal Artillery), were those designed by the late Lord Armstrong ..."
Two quotes from above from the 'Textbook of Service Ordnance 1923', the capitalization within is my emphasis.
From the above, it can be seen that the term 'Gun' is not restricted to, or dependent upon, an absence of rifling. So the statement that 'Gun refers to a smoothbore' is, as demonstrated above, incorrect.
So, some examples of the official use of Gun/Guns when referring to artillery pieces that have rifling:
Pamphlet title: 'Gun Drill for QF 25 pr Gun, Marks 1/2, 3/1, 4 on Carriage 25 pr Mk 1 (1960).
Or maybe 'Gun Drill for QF 18pr Mk IV Gun on Mark IVP (LP) Field Carriages 1939',
or perhaps 'Range Tables (Part 1) for QF, 25-pr Guns, Marks 2 & 3 (1953).
So, a range of dates and titles, all referring to artillery weapons - sorry, 'pieces' - with rifled ordnance, as 'guns'.
Cease fire ... target destroyed.
Mike