I doubt a 3/8" bearing, or even two of them, could do what you want...the load at the turret is going to be very high, because of the length of the arm and the weight. You might see if you can use a
"lazy susan" type bearing under the sprocket, and maybe a 3/8" bolt going thru the bearing in the center to hold it down to the robot frame.
Even this is probably going to be kind of weak for what you want to do.