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A spring-gun 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.
Although there have been few reported cases of use, there have been several unconfirmed cases over the 20th century. The obvious implication is that spring-guns are still in use today, especially in circumstances where property of high value is in a remote location that makes other forms of securing it unreasonably difficult to effect.
Spring-guns are often used to protect premises from trespassers. For this purpose, spring-guns are often placed in busy corridors such as near doors. A trespasser opening the door completely would then be shot. Residents who are aware of the trap use a different door or open the door halfway and disconnect the tripwire. To reduce fatalities by using this trap, non-lethal calibers are often used, or the spring gun is fitted to fire less lethal ammunition.
For example in the United States, most spring-guns are loaded with non-lethal caliber or shot to avoid liability arising from the use of deadly force in protection of a property interest. Posting clear and unmistakable warning signs as well as making entry to spring-gun guarded premises difficult for innocent persons, such as high walls, fences and natural obstacles, are significant ways to reduce potential tort liability arising from the spring-gun's wounding of a careless or criminal intruder. Important US lawsuits regarding trespassers wounded by spring-guns include Katko v. Briney. Bird v. Holbrook is an 1825 English case also of great relevance, where a spring-gun set to protect a tulip garden injured a trespasser who was recovering a stray bird . The man who set the spring-gun was liable for the damage caused.
A historic use of a spring-gun occurred during the night of June 3 or early morning of June 4, 1775, when a spring-gun set by the British to protect the military stores in the Magazine in Williamsburg, Virginia, wounded two young men who had broken in. The subsequent outrage by the local population proved to be the final act of the Gunpowder Incident, leading Governor Dunmore to flee the city to a British warship and declare the Commonwealth of Virginia in a state of rebellion.
Another case is McComb v. Connaghan in which a 19-year old burglar was killed by a spring-gun that was set up by the property owner who was a repeated victim of burglary.
Alternative traps are mines such as the crowd control munition, gas mine or the directional mine, such as the SM-70, which was used on the inner German border to prevent refugees from escaping communist East Germany. Crowd control munition and gas mines can be less lethal, while concussion mines are meant to kill. The latter are thus only used in military perimeter defenses.