Yeah I see the problem...I think it has something to do with the runtime not deleting objects in loops or something. If you need a quick work around for now, you can add a private variable to bullet 'isdestroyed' and compare it to 0 (and set it to 1 when your destroying an object) on loop 'move'. This will make your game behave as you expect.
btw nice use of lights
I love how the screen flashes when you fire. One small suggestion is I noticed the turret and the tank are out of sync. Put the gun - set position to playeranimation below the playeranimation: set position to player and then the turret wont lag a frame behind the tank
Oh and nice use of an action point to move your object in the direction it's facing
I cant wait to see the completed game!