http://www3.towerhobbies.com/cgi-bin...?&I=LXDS62&P=7
The link is to Tower and shows a Proctor pulley. There are several other types too. You could use three or four of these to support an aluminum ring upon which the turret, gun and gunner would be mounted for easy turning. Powering it is another matter but making a free turning assembly is half the battle.
Personally I would do as gary9648 suggests with a mini servo turning a rubber wheel which would run against the ring. The servo/drive wheel would be on a pivot with a rubber band to provide tension.
I've seen the plans for the CD Scaledesigns C.1 and they are FANTASTIC. It will be a really great model.