Cool game! I already played a submission with this very concept, but I have to say that I think you have executed it better. For example I really like the arm, it is a cool addition. This way the bullet can't get stuck behind spikes or an enemy. I also really like the concept that you have to pay attention to always pick up your bullet at the end of the levels. One minor complaint is that enemies seem to be damaged by the bullet even if it is not moving. But if it is intentional it is fine.