I keep seeing three basic turret designs. Which way are you going?
The Pallet Carousel as mentioned which is $400+ delivered.
The floor mounted pivoting support. Usually a welded up axle bearing mounted to the floor. A chair attached to it so the gunner rotates with the turret and turns it using his feet freeing up both hands for gunning. From the back of the chair, the turret support goes up and the turret frame welded to it.
The rest are some type of track where the turret rolls around on. Kind of like the pallet rack but cheaper to build in theory. The problem usually being making something ROUND that is 40 inches or so in diameter.
This is what I researched to use. Round clothing displays.
When you go to the website they have 42" add on rings for $32. "g" in the image. The bottom ring.
Went to the mall and wandered around to look at some. Made the mistake of wandering into Victoria's Secret and the girl asked if she could assist me. Told her I was just there to look at their racks.... She misunderstood...
Anyway, they looked very usable for a turret track... Some V rollers top and bottom times 4 of them should do the trick. My original application would be a camper shell turret and I would drive the truck to the game. Thus I would have to remove the turret for the road trip and have some assembly required at the game. Otherwise I would have TJ's problem of getting stopped by the cops every 30 minutes driving down the interstate. So I was designing a quick install/uninstall turret. Unbolt two of the bottom rollers and slide it off the ring.
I went to see the tank on tank battle at OK D Day. Watched a tank hit a hole and bounce the turret off the track. The only thing that kept it from falling completely off the vehicle just happened to be the gunners body that got pretty chewed up before they got stopped and got it off of him. It wasn't that light of a turret. I decided a LIGHT turret that is mostly netting was the way to go. And the bottom roller or at least a safety L bracket to keep it from bouncing off.