A gun that is set to fire on any intruder that comes in contact with the wire that sets it off WordNet 3.0 & Open English WordNet

A spring-gun, booby trap gun etc is a gun, often a shotgun, rigged to fire when a string or other triggering device is tripped by contact of sufficient force to "spring" the trigger so that anyone stumbling over or treading on it would discharge the gun. Wikipedia

a gun, often a shotgun, rigged to fire when a string or other triggering device is tripped by contact of sufficient force to "spring" the trigger so that anyone stumbling over or treading on it would discharge the gun Wikidata

A gun rigged to fire when a tripwire is crossed. Wiktionary