We have seen DIY bullet speed measuring systems before, this DIY Paintball Gun Chronograph by Instructables user KLASH69 has been well documented and includes schematics, PCB layouts and code so you can get a jump on building your own.
“The operation of this chronograph is rather simple, there are two IR transmitters and two IR Detectors, each transmitter/detector pair is placed exactly four inches apart.
When an object breaks the first beam the microcontroller starts a timer (Timer1). This timer will keep running until the object breaks the second beam or the timer causes an interrupt (on the 65,536th instruction cycle). If the object breaks the second beam before the timer causes an interrupt, it will stop the timer and read the 16-Bit value in the registers TMR1H and TMRIL. We can then calculate the amount of time that it took the projectile to travel the length in between the IR beams, which are 4 inches apart, and find the speed of the projectile.”