When the lab technician wakes up when Brooke is controlling the zombie patient he fires his gun into him and unloads the chamber including a few clicks to mean he's out of bullets. Moments later when Brooke breaks free with the help of the lab technician who is now a zombie and whom she is controlling, she grabs his holstered gun and shots him in the head. He had no time to reload therefore the gun should be empty.
When the Doctor is attacked by a zombie for the first time he clearly empties his gun and puts it back in the holster. Shortly after, when Brooke gets his gun, she shoots him with it even though the weapon is never reloaded.