Hi - and welcome aboard! Several solutions to this problem come to mind. There is at least one manufacturer producing aftermarket turned brass barrels on 1/700 scale. It's called Clipper Models, and the range is considerable - though not cheap. A quick look through the usual websites only found one retailer that sells Clipper Models products: Pacific Front Hobbies. A set of barrels to furnish one ship seems to run around $15.00 or thereabouts.
Skywave, the excellent Japanese company, sells several sets of detail parts for 1/700 warships. Most of them concentrate on smaller weapons, but one of the Skywave sets does contain barrels (and turrets) for Japanese battleships. I believe the same box is sold by Tamiya. Some of the barrels in those sets probably could be modified pretty easily to work on warships of other nations.
For a good, comprehensive overview of the wide range of detail parts available, take a look at the Steel Navy website (<www.steelnavy.com>). It contains quick links to the sites of various manufacturers and distributers. Two of the latter that would be especially relevant are Pacific Front Hobbies and The Naval Base.
To make your own barrels really would require a lathe - probably a pretty sophisticated one. Gun barrels on those scales are mighty skinny and mighty long - and since they sit in pairs and threes quite close together, any inconsistency in them will be immediately obvious to the eye. My old-fashioned Unimat lathe might be barely capable of such a project, but it would take me quite a while. (The computerized machine shop at the community college where my stepson is studying machine technology probably could whip out a set of gun barrels in an afternoon, but that equipment costs thousands of dollars.)
The most economical solution might well be to buy a plastic kit with similar turrets and canibalize the relevant pieces. The recently-released kit of the U.S.S. Arizona, by Dragon Models, contains nine exquisitely-turned brass 14" barrels, for instance, and the new Tamiya U.S.S. Missouri has plastic 16" barrels complete with blast bags. Another point: such a kit probably would yield all sorts of other parts that would come in handy for a scratchbuilding project.
Hope some of this helps.
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.