Sheesssh! Everyone is working on the same thing. Great idea. Bryan Litz uses something acoustic.
I've been playing with the same thing.
Way different approach. I tend to way under engineer.
I've put together a system that works in the apartment
as a proof of concept.
Using a 5 buck blowgun with darts as a test device. (wife is about to have me committed - need way more space!)
Components include the following LEGOs components
Sound sensor (microphone)
Piezio sound sensor (fasten to target)
Problems: Down range detection information back to the bench.
Easily fixed with WIFI.
Deal breaker: NXT Mindstorms only counts milliseconds. Not nearly accurate enough for high velocity projectiles.
However, there is a gps on the market that is configured to output microseconds which would, over a period of time, provide for a distance value more reliable than a LRF in addition to the accurate time stamp.
Thus: Time of impact - Time bullet exits muzzle = Time of Flight.
Arduino with GPS recording time to the microsecond level. WIFI to xmit back to host Arduino. Increases base cost by 80 bucks.
Another way is use Arduino and piezio sound detector "super glued"
to target. WIFI to xmit pluse to stop count at the host.
Start count - Stop Count = microseconds travel time.
Sure wish it were that easy.
If yur into programming (C++ or similar) this is straight forward and quite affordable.
I'd jump on it but I have too many other irons in the fire..............