I just tried this and it was a bit finicky.
The precision was not very good; it would go from 300 to 400 to 350 to 300 again, etc.
In the end, they averaged out to around 330, whereas the XCortech said 360. I used .25 bb's. I expect that the average I got was based on pure luck, though.
Other people should experiment on their own to see if this works for them.
I would recommend the led/photoresistor homemade chrono though. It seems like it would be a bit more precise than this one. You just attacht he photoresistors to an audio cable in the same way as this and then attach the LED's to a power source and mount the PR's and LED's on opposite sides of a tube. It should work relatively well. Keep in mind that I actually don't know if this will work as I haven't tried it yet; I just thought of this a couple days ago.