I can't afford a chrony but have been looking for how to do a speed test, I found this web site after countless hours of searching on how to make your own -shooting chronometer. all you need here is a PC and sound card and some free software. Enjoy!
Think it would help to attach led lights above the phototransistors? It Should act as a projector and cast a bigger direct shadow of the ammo the closer to the led you shoot? Also probably make it more possible to shoot at dark places.