Pivot Point Reassignment

So I have this minigun model and I need to change the pivot point of the top objects to be in the exact center of the disk just above the base. It needs to be exact so that it rotates properly. How would I do this?[3660-turret+help.png|3660]

I am sorry about the big white spot to one side, I have a duel monitor set up.

Okay So the sphere on top is the current pivot point. The sphere on the bottom is the place where I want the pivot to be. Keep in mind that it needs to be in the exact center of the second sphere. ANY HELP WOULD BE AWESOME. THIS IS ONE OF THE MOST ANNOYING THINGS I HAVE EVER EXPERIENCED IN UNITY.

The usual solution is what @Fattie said: create an empty object and child the model to it. This makes a composite object, where the parent is the actual weapon (the one you will rotate, move etc.). Adjust the model position and rotation to reach the desired results - as a child, it will obediently follow the parent without loosing its relative position.