I would think that the gun (and trigger puller would ) would be propelled in the opposite direction forever unless attracted by a nearby by celestial body...the irony is that we would be more attracted to the more massive bodies, as counter-intuitive as that might seem.
The bullet will meet the centrifical force of the opposing atmospheric space minus all gravity divided by three with equal maass of the bloated brains of many scientists who think they know so much! Ha!