Love the game, but the rail gun is awful. Whether firing into empty space or passing harmlessly though an opponent...it is easily the worst of the weapons. For sequels...consider allowing click-fire for the rail gun, so the shots aren't constantly wasted.
Not being able to aim and see the target at the same time is needed at first few levels, in order to get a feel for the physics. Otherwise, it just becomes a game of 'guess'.