couple of questions

long range, medium range or short range aa weapon? what country? at the muzzle of the 16" or against the turret face?

full aa defence really so all of em :)

country doesnt really matter

turret face is fine at 700 scale will it even be noticable if i use a straight pipe?

Straight pipe will not be too good.  Barrels are easy enough to come by. eBay is usually a good place to buy $5 junk kits.

cheers i cant find any real cheap bits i might have a look for some smaller 250 scale guns and made turrets

Best Surface-to-Air in '70s is a toss-up.

Best P(k) probably from Talos--but, luckily, we have no engagement data to base that upon.

Terrier probably wins from a reliability standpoint (Terrie becomes the Standard later on, in a generic sort of sense).

Next best is probably one of the 5" 54 DP mounts--although those are really just follow-ons from the 'best WWII AAA" weapon, the 1946 3"50 DP.

By 1970, the best anti-air weapon is a squadron of F-4J or the same quantity of F-8.

All naval rifles have tapered barrels--the 16" 50cal are no exception; and those have a lot of taper, along with some "steps" in the profile.

For sources, you can always look to some of the 1/720 kits for those big barrels, even the turrets, as long as you do not get caught up in the turret shapes.