Aye though I'm thinking that you're using some sort of laser/sound trip to fire the shutter at just that moment. A sound trip though would have to be very well placed and setup so that it wouldn't trip when the airgun fires - so I'm thinking a laser trip set to fire when the cap breaks the beam with the laser being to the right of the drop and a distance set so that (taking into account delays as well as the speed of the cap) the cap is hitting the drop location as soon as the flash fires (shutter either in bulb mode or also tripping the shutter at the same time).
Tricky part I think would be firing the gun at the right moment to get the cap to hit - unless you get very extravagant that's a good bit of aiming and timing of shots
Hmm bulb mode then I assume - with either a totally dark room and flash being the only light or perhaps a custom built external shutter whilst the camera still runs in bulb mode (similar idea to the first only it allows one to work in a more lit environment).
Still interested in how you triggored the setup though -