Hi,
I'm looking into making a chronograph to measure an airsoft rifles output. The speed ranges from 300-500fps, with up to 60 rounds per second. I would like to measure projectile speed, as well as how many rounds fired. I was looking at this photo interrupter (Photo Interrupter - GP1A57HRJ00F - SEN-09299 - SparkFun Electronics) from Sparkfun, but I'm not sure if it would be best suited for the application. Is there a better product available? Guidance would be appreciated.
You need two (you knew that), and you need to do some simple arithmetic based on the size of the projectile and its velocity, to see if it will obscure the sensor for long enough for the processor to register its presence.
I'll be listening into this, but for paintball use. 
I do own a comercial chronograph (Shooting Chrony for real bullets) and as far as I can see, it uses photo sensors, but just for reflection. That means, emitter and sensor are side by side, both pointing upwards. Something quite similar to this one: Infrared Proximity Sensor - Sharp GP2Y0A21YK - SEN-00242 - SparkFun Electronics. When the bullet flys over them, light gets reflected on the bullet and the sensor detects it. Doing that with two identical sensors, separated by a know distance, the rest is just math.
You will also need some kind of IR light filter to avoid noise and "false alarms".
This way, you dont risk shooting the sensor (or at least, is not that easy).
For exclusive paintball use, I've seen other products that work using doppler, but I don't have the smallest idea of how they work (but would love to 
).
AWOL:
You need two (you knew that), and you need to do some simple arithmetic based on the size of the projectile and its velocity, to see if it will obscure the sensor for long enough for the processor to register its presence.
Yes, I knew I needed two sensors. My plan is to mount the sensors in a tube, so hopefully ambient light will not be an issue. I can only mount the sensors a maximum of 10cm apart. Is this distance far enough to provide decent accuracy?
The window is 1.8mm wide, and the projectile is 6mm (edit: more accurately 5.93-5.98mm) wide, leaving a total of 2.33mm of BB to obscure the window (from front edge of BB on back edge of sensor to back edge of BB on front edge of sensor). Given a top speed of 500fps (152.4m/s), the BB would completely obscure the window for a total of 15.29 microseconds. Am I thinking along the right lines?