Got my webcam sorted out, and after a few other software and hardware hurdles I have a more-or-less functional airsoft sentry gun. It needs a good deal more calibration, but it's good enough to hit big targets. And I forgot how much airsoft pellets hurt!
Here I am working out a few bugs with my target stick and the gun in the background:
Here are a couple of videos of the system in action:
video 1 - this is the first test. It misses right a lot, but it tracks and fires.
video 2 - it hit me most of the time in this video
video 3 - it hits the balloons about 3 times, but misses right a lot. It also doesn't break the balloons. I have some higher-powered gas that might give let it pop them.
So, there's still a fair amount of fiddling and tuning and calibrating left to do, but I am super happy with where things stand after just 3 evenings of work. I also have already learned a lot about jMyron and video capture and processing in Processing, which was a big goal for this project. So far so good!