I think you may have a more fundamental problem, but I'll return to that in a moment.  Presumably what happens is that when the tank is at an angle the shots fire from a location other than the tank.  What you should perhaps consider is that the translate(-300,-200) takes place after the rotation, and is therefore affected by the rotation: i.e. the whole coordinate system has been rotated so translate(-300,-200) does not return to the same point, but a point in relation to the angle of rotation...  Perhaps you wanted to use pushMatrix

Of course the more fundamental problem I alluded to is that you'd need to store the angle each shot was fired at and rotate appropriately before applying motion to it; otherwise I suspect the shots will continue to be affected by the rotation of the tank after they have been fired.  That would involve a whole lot of push/popMatrixes which doesn't seem ideal...

A better option would be to calculate the direction vector for the angle when fired and avoid using rotate() altogether.  i.e. if fired straight up vx (velocity on the x axis) = 0 and vy (velocity on y axis) = -1.  If fired directly to the side vx = plus or minus 1 and vy = 0.  Any angle in between would need an appropriate vector (trigonometry is your friend).

I know that would be my preferred approach as relying on rotation of the whole coordinate space is likely to make collision detection rather tricky...