Have you ever considered a laser snap-cap and a hit detection software package?
I am an application developer and at one time considered making my own, so my first step was to research it to see what already exists in the market.
The general idea is to use a snap-cap that generates a short laser dot upon being struck by the firing pin. An application running on a laptop uses a webcam to take a continuous series of bitmap samples while pointed at the target area. A detection algorithm is then used to isolate/find any potential "bright red dots" and record their x/y coordinates within the target area.
The nice thing about this is that you get to use your actual firearm and it is completely passive. The timing reaction and location of shot can also be recorded very accurately.
Here is an example I found on the web, it may have also been mentioned on this site before at one time:
http://www.centrolutions.com/products/lasr-laser-activated-shot-reporterI am not associated with the above link and have not tried it (i.e. cannot endorse), but it will give you the general idea. Not sure this would work for what you are looking for, but I think it looks pretty neat.
Thanks, Mike