Here is a 12g adapter and .209 blanks - they fit into gate mines really well.
Cheapest place to get gate-mines inc postage is airsoft world
I haven't gotten around to using mine in the field yet. I've actually misplaced the poxy thing in 1 of my piles of gear! Still, i have a few cunning plans and one of them will take some preparation which i have yet to do.
The thing with the old unpinned 'nade spoon trick is that it makes a noise and, depending on what you use as tripwire, the victim may hear the sound, see/feel the tripwire, and dive for cover, because it doesn't go off for a few seconds - you don't have to be Usain Bolt to to clear 5m in 2-3s
A few of us were talking about electric dets a while ago. Of course you can get radio controlled ones, but they're not cheap and by all accounts not totally reliable - which doesn't surprise me: it's an axiom of sound engineering that the best radio system is always almost as good as a wire...
But there are other options, the simplest of which really appeals to my sense of aesthetics: if you take a strand of steel wire from a brillo pad and connect it across even a low voltage, like 3.7V, it will make a 25mm length of it glow orange - which is enough to ignite the striker of a pyro - but there's still the problem of the victim hearing it and getting away... but, if you drill a hole down the striker and get the ignition element right inside bypassing most of the fuse, it will go off much faster.
Naturally making set'n'forget traps with an electric det is much less likely to be noticed, because you don't need much mechanical force to set them off: a spring loaded switch like a wooden clothes peg with a foil contact on either side of the clip, with a slip of plastic as an insulator to hold them apart. Attach a piece of 10lb black fishing line (which is invisible against anything other than a solid colour background and practically invisible even then) as a tripwire to pull the insulator out... boom!