Today I built a prototype Chronograph for Airsoft and Paintball (and Nerf!) using an Arduino and IR LED emitter/detector pairs, I am not sure it it will be fast enough for measuring airsoft pellets though. The initial code was pretty bad, it could only measure a maximum of about 5 fps! (Too much serial debugging going on)
Here is a picture of it, I will post more once I do more testing and debugging.
The default ADC sampling rate is around 120us per sample which is not good for this project.
Thanks to some other blogs about high speed ADC on Arduino (that I have referenced in the source code) the system is now sampling at 15 micro seconds and using interrupts for better performance. I think 15us is good enough, but in case it is not I have ordered some IR detectors from amazon (Uxcell a14060700ux0143 SM0038 3 Pin Ground Vs Vout Infrared IR Receiver Module 250mV) that will allow me to use the digital inputs at 16MHz.