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Spring-gun

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.

Usage examples of "spring-gun".

We Noifs have a spring-gun in our heads, so to speak, and it solves each problem as we meet it, but each time it jacks up into position a new problem.