A turret includes a rotary bearing with an inner race, an outer race and a central axis of rotation. An internal tooth drive sprocket is fixed to the outer race of the bearing and centered on the central axis of rotation of the bearing. A turret plinth is fixed to the inner race of the bearing. A drive mechanism for rotating the turret plinth with respect to the internal tooth drive sprocket includes first and second sprocket assemblies each having a pair of upper and lower external tooth sprockets fixed to a shaft. The middle chain row of a triple roller chain engages the internal tooth drive sprocket. One of the outer chain rows engages the upper sprockets of the first and second sprocket assemblies and the other of the outer chain rows engages the lower sprockets of the first and second sprocket assemblies. Hand wheels connected to the shafts of the sprocket assemblies provide input motion to the drive mechanism.