A snare gun was a weapon which was used by the Graske. Inside a snare gun there were snare cables, which the Graske shot at their victims to teleport them through time and space. When they teleported a victim to the Limbo the victim's existence was whiped out completely. Noone except the Graske could remember the individuel anymore, it was like it never excisted. An exception were only the owners of the Verron puzzlebox, which were protected by it and could remember the victim. A snare gun could also be used to get back someone from Limbo, which would correct the changes in history.
Alan Jackson used the Snare gun to get back Maria Jackson, who was trapped in Limbo. He also tried to get back Sarah Jane Smith.(TV: Whatever happened to Sarah Jane) However Sarah Jane inhabited a paralel reality, so she could only be seen as a reflection on the mirror in Sarah Jane's, or in this reality Andrea Yates', attic. (TV: SJAF 1)