[asid61]

It's a turret for a t-shirt cannon. Basically, the actual barrel rotates on something like what the arms this year had (stright piece up + angled claw) to be able to aim and shoot better. It's also practice for future years.
It doesn't weigh that much but due to the design which incorporates bevel gears, there is a large bushing on the bottom coaxial with a shaft going to a bevel gear to rotate the barrel up and down independent of the module's rotation. So the turret experices very slow radial loads in the 100rpm range but around 30lbs axial.
The load is on the flange of said bushing.

I looked into ball bearings in the 3" range, but they all cost more than $100. I also cannot design for this bearing to be smaller due to some interesting design issues that have to deal with a cim mounted to the bottom of this turret.