Thanks AWOL. That's an interesting idea. I've not considered radar, because I thought it would be too expensive, but I just did a quick search and found some inexpensive units. I'm not sure if the off-the-shelf units will fit the bill, but it's another approach I'll consider.
Thanks cyberteque. I have an inexpensive chronometer for speed, which uses optical sensors. The electronics detect the slight reduction in light when the bullet passes over it. I hadn't thought about using that (or the same method) as a trigger for the system. Good idea.
Thanks cr0sh. I looked into linear CCD arrays early on and set them aside due to cost, speed and size. I don't remember all the details, but the longest ones were only a few inches with far more resolution than I need. I suppose I could use a lens, but I think it will be easier to space individual detectors out over a longer length. I'll keep it in mind if I run into trouble and have to find another way. Plus, I'm sure they're getting cheaper all the time.
Thanks terryking228. Your questions / suggestions have spurred a re-thinking of the project. I think I can do it with one continuous source (LED, Halogen etc) and a reasonable number of detectors on the opposite side (multiplied times 4 sides). I'll have to do some cypherin' and tinkerin' to see if I need to latch the signals or if I can read them fast enough, but that is straightforward enough.
Any more ideas, thoughts or questions, please keep 'em coming.