This is my knex land mine that shoots out shrapnel when a trip wire is pulled. i decided to use the simple block trigger idea and make something creative instead of a gun (although you may consider this a gun, because it shoots knex pieces) any ways, you can use pretty much any knex grenade that fits itself together loosely, like mine(you just have to put a white rod on it).

Step 1: Firing Mech

This is the Firing mechanism that makes the grenade explode. since the bullet doesnt go all the way in the barrel, it smacks it and makes the grenade blow up. Make sure to tape the firing pin. adjust the rubber bands so that if you pull it back, put it at a 45 degree angle, load in a white rod, and shoot it, it should go 15 to 20 feet. here i used 3 thin rubber bands.

Step 2: Stand

this is the stand that holds everything up. make sure to put something small but semi-heavy on the leg opposite of the trip wire. (in my case, a rock band drum pedal.)

Step 3: Grenade

this is easy, just build it. you can use any knex grenade that fits together loosely like this just ad a white rod

Step 4: Tie to a Doorstop or Some Nail Sticking Out of a Wall or Something

I suggest tying it to a door stop with string and when someone opens the door it goes off. its less noticeable if you put it there and gets you by surprise.
to load it, pull it down the firing pin and load in the grenade. when the trip wire is pulled, it goes off and scatters shrapnel everywhere.