This was a design used by the Viet Cong in Nam. It's very effective.
You need:

1) Shotgun shell
2) squares of plywood, about 2'x2' and 1/4" thick
1) nail

Dig a hole big enuff for one piece of plywood to fit into. Drive a nail
through the center of one sheet of wood, and put it in the hole, nail
pointing up. Now, fill in dirt around this sheet, until it is mostly covered,
and only the tip of the nail is still exposed. It should still be below
ground level. GENTLY place the shotshell on top of the nail, and fill in dirt
around it, so it is supported on top of the nail, and held in place.
GENTLY place the second sheet of wood on top of the shotshell, and put leaves
and such on top of it.
When someone walks over it, they press the shotshell onto the nail, and it
blows right through the wood, and takes off their leg.

Wait....have you TRIED this one?

Cause I mean I don't see how a nail would have the force to set off a shotgun shell. But maybe I just don't know sensitivity of certain shells.

well no if it hits the blasting cap at a normal walking speed it make loads of sense.

tho obviously it isnt very reliable.

where the hell would you put this to make someone walk right on to it? if they step 2 inches to the side of it nothing happens... seems kinda inneffective unless you make like fuckin 20 or 30 and puttin em in a football field or something

well putting them in a football field would work or u could place multiple shells on on board kinda like a six on a dice

This wasn't used in nam because it doesn't work. Without a metal chamber (such as in a shotgun) surrounding the shotshell, when fired it will barely have enough focused force to pierce the wood, let alone 'take your leg off'.

Even if it did, the kind of force you get form stepping on it would just tip the shell over. I don't like you, you're a fucking retard.

this is one of the stupidest things ive ever heard

go die..