Had a look at the video. The jerky turret rotation should be dialed out with full trim down on the elevation channel. Also the link I mailed you to the Turnigy servo controller would be really good in this application. Replace the servo reducer with this other controller, and you will have the ability to reverse the servo and slow down its speed of operation so the barrel moves smoother. Should also help with that issue you were having with the Barrel moving when your fired the cannon. Although in this video it seemed OK.

If you buy a DPDT center off toggle you can wire this into the radio, and have your weapons control on one toggle. I can show you how to wire it up.