My idea is to have a expansion chamber that is filled with a shreader valve that has a burst disk held in with a coupling. A free moving cap would go on top of the burst disk and would hold the bb's/powder. Inside the cap would be a permanent nail or some sort of sharp object to pierce the burst disk when the whole "grenade" is agitated.(when it hits the ground) To make sure that the grenade doesn't prematurely get agitated when you are carrying it a saftey pin would be located on the cap.
The cap would be held on by String or maybe a rubber band so that when the grenade fires the cap doesn't come flying off and kill somebody. Since it would be held by string or rubber band it would partially open and the bb's/powder would obviously follow the air out the sides.
To better show what I am talking about I drew up a little model to show my idea. Hopefully tomorrow I can build a quick prototype to see if this will work.



Note: The string that keeps the cap on isn't shown in the pictures.
If this prototype works then you could attach a wire to the pin and make a tripwire mine, add some paper, kleenex or whatever in between the cap and the coupling to add more friction for a grenade or even drill holes on the top of the cap and fill with powder for a step on landmine.
So what do you guys think?