Conservation of momentum, p = mv
Velocity of shooter =
(Velocity of bullet x mass of bullet) / (mass of shooter)
The shooter will be propelled in the opposte direction of the bullet, but at a much slower velocity because he is so much more massive.
I have no problems with a gun in space, there are already so many different ways to kill people and yourself there. Besides, if they land in Miami they're going to need it.