For some stupid reason, you can't just go out and buy yourself a 40mm grenade launcher nowadays, which is a real bummer for enterprising civilians who want to play around with explosively launched single use video cameras. So it's a good thing you can just go out and buy a 37mm flare launcher instead.
I don't know how much of a difference 3mm can possibly make, but when it comes to launching projectiles from handheld, uh, launchers, apparently it's the distinction between what you can screw around with and what you can't. A couple hackers at Defcon decided that they were going to try and use a 37mm flare gun to launch a video camera hundreds of feet into the air, partially for fun, but also because it would be a really handy way for civilians (i.e. local police) to scout out hostage situations and such.
To get the system to work, a little wireless video camera is embedded inside a hard plastic cartridge, equipped with a parachute at the top and a powder charge at the bottom. When fired, the camera flies up to 500 feet in the air and the parachute deploys, giving you an eight second ballistic aerial view of whatever you were pointing at. In theory, sounds great, except that during the first test of the launcher, half the charge failed to ignite, the wireless didn't work, and the parachute incinerated itself. Sweet!
A few months of extra work will likely get the 37mm camera launcher up and running, and eventually it may boast capabilities similar to the system it was based on, a 40mm version for the military manufactured by an Israeli defense contractor and already in active service. This whole project exemplifies one of the great things about the hacking and DIY mindset: the military has a neat toy they won't share? Fine, be that way, we'll just go build one ourselves. And you know what? It'll probably end up being several orders of magnitude cheaper. So there.