Step 1:

Build your object inside UnrealEd using the 'normal' methods.

Step 2:

Place a 'normal' cube over your object for intersection. Place the PivotPoint over the exact position of the wanted 'rotating spot'. This is the point where all rotation will take place. In addition its the center position for spawning projectiles if your CRM should become a weapon! Don't forget to take a look on the cubes rotation (red arrow)!
Now, intersect the brush (CTRL-N).

Step 3:

Select the ControlRotatingMover inside the classes window

and transform your brush into a mover by pressing this button:

Step 4:

All that's missing now is something that controls the CRM. Insert a trigger inside your map to control it. If you want to control your CRM from a position not directly behind it, you can use a ViewSpot to set the controlers 'virtual' position. Please keep an eye on the ViewSpot's rotation and set its AttachTag (submenu Movement) to the Tag of the CRM. Use a trigger to 'trigger' both.

DoubleCannons NEW (v2.11):

The CRM feautures Double-Cannon capability and a new FireOffset. If you want to build a double-cannon you don't have to build TWO CRMs, only ONE is needed!
In the submenu ControlSpecialFire you have the chance to enter a FireOffset. It's similar to the one used in the weapon classes. The old FirePositionOffset (still existing) only moves the projectile into the direction of the current rotation. Now the new FireOffset can be used to move the spawning position of a projectile or instantHit to any position you specify. Normaly you'll use only its y-value to move it to the right or left.
In addition you can set the bDoubleFire parameter to True. So two projectiles or instantHits will be spawned at the same time. The second one is spawned at the y-value * -1.
So, you get a double cannon with just ONE CRM !!!