This guide shows you how to make a shot gun trap (this also works with any other gun).
Grab a button.
Spawn a human on top of the button (to see how far you have to put the gun of your choise) then spawn the gun and freeze it facing towards the guys head.
Add the activate wire (i forgot what its called) on the button to the gun.
Attach the button the the floor securely.
Take the guy off the button and place him back on it and watch the fireworks.