Most target pistol bullets are subsonic, so detecting the 'sonic boom' isn't feasible. I hadn't thought about detecting a shock wave from a subsonic bullet. I'm not terribly optimistic about it, but it's worth a try.
Most commercial electronic scoring targets use a plastic film and detect the sound of the bullet breaking the film. There are microphones in each corner and triangulate from the time difference between the 4 signals just like the referenced thread is trying to do. It works well, but after each shot the film has to advance so there is a clean spot for the next shot to hit. Needless to say they go through a lot of the film, which has to be bought, changed and disposed. I am hoping to achieve it without expendables & waste.
I've done quite a bit of thinking and modelling of light curtains. I have no doubt it would work, but the simplest solution I've come up with would need many photo-diodes and (probably) an FPGA or high power micro to deal with the large number of IO pins and speed required.
Terry - I'm not sure whether to be impressed or horrified at the thought of holding a necco wafer and letting someone (even a great sharpshooter) shoot it 