I went the pyrotechnic route with my design. It utilizes a large firecracker with a friction fired fuse i designed to light it. The fuse is basically a drinking straw with several matches taped to it. The strikers from the matchbox with a metal ring attached to them are held onto the end of the matches with an elastic band. When the ring is pulled, (say by an attached trip wire), the matches on the straw light, igniting the firecracker.
The trap itself was the above with about 300 .2s duct taped around it, in effect being similar to a hand grenade staked to the ground. I got bored and decided to make this thing and test it, The test, basically stand in front of it and pull the trip wire. Results: Noisy, but perfectly safe. i was hit with probably 20 or 30 bbs and was standing about 6-7 feet from it. From the force of the impacts, I believe the bbs were moving at probably 200 to 250 FPS (felt like gettin shot with a cheap springer). The spread covered the room i tested it in which is about 400 square feet. Outdoors this would probably equate to about a 30 foot radius at which the bbs are still moving at a decent pace.
All in all i think it worked pretty good.
Travis R. (aka Trapper)
Canadian Airsoft Marines. Field owner