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I've tried out the commercially available simulators and they are awesome. I have everything needed but a web cam, so I'll set this up if I have some time.

Haven't tried it yet, but Projekt ARES is a DIY laser shooting gallery. Software is free, runs on a PC hooked up to a projector and gets the shot indication via a cheap webcam. You need a laser cartridge/gun to get started.

Projekt ARES website

It looks a whole lot like shooting simulator ranges that cost thousands of dollars, but with a European flare.