If you're going to use Physics on the cannon, you should use physics to turn the cannon. It's as simple as that.
If you move the cannon in any other way that doesn't use physics, you are going to get strange results.
When two physics objects collide, the physics engine is doing the colliding. It is calculating the force and the acceleration and the collisions within the physics behavior. When you move a physics object another way, by changing the angle or moving it with Drag and Drop or something, then the physics engine doesn't know what is going on. You can push two physics objects into each other, which is something that wouldn't normally happen in the physics engine. Also, if you manually move an object then there is no acceleration or force applied to it in the physics engine, so you won't get accurate collisions.
Long story short, physics doesn't work well with other behaviors or movements. Physics objects only work properly with other physics objects. Hope this helps.