Well, Scratch, keep wondering. What you have described is known legally as a "spring gun". In every state I know of, setting one is a felony with a LONG prison sentence.
The original spring guns were swivel mounted guns, with a string run at knee heighth- walking into the string caused the gun to swivel towards you and fire. Set by the head Ghillie to take care of the bleedin' poachers that were after the Laird's fine fallow deer (or rabbits, or grouse, or salmon)