We built an award winning & published turret system in 2006.
http://www.chiefdelphi.com/media/photos/33185
http://www.chiefdelphi.com/media/photos/25671
It was a custom lazy susan application with two plates. One on top of the other & a LOT of small plastic ball bearings riding in a grooved track between the two.
A motor driven wheel (shown below the plate on the left of the image) drove the top wheel & allowed everything on top of that including the motor driven shooter wheel to turn with ~340 degrees of motion.
The only reason we couldn't turn a full 360 degrees or beyond is because we had cables running up to the shooter motor we couldn't safely route around the turret as it spun.
The ~340 degrees was plenty of a range of motion we desired though in this instance.
If you can get a copy of the FIRST Robots: Behind the Design book from 2006, it is featured in there with a more in depth explanation.