It seems that your own sundy is higher compared to others, causing you to shoot your own deployed dome if you sit in the guns. If you use someone elses sunderer with deployed dome it is lower allowing you to use the guns freely. I asked other players and yes their OWN sunderer is higher compared to mine, you can see the deployed bars at right and the wheels sinking into the ground this is on every single sunderer that is not yours. Also the dome is ignored sometimes, not sure why but it seems that at high ranges the guns completely ignore the shield.