jackrae:
Suggest you might want to think about using a sound sensor to listen for the shock wave that the passing bullet creates. This then does not rely upon accurate "aiming" of your proposed photo detector.
The shockwave trails behind the bullet (and the faster it is, the more it trails). In a lab environment where there may be only 6 to 10 feet between the rifle and the bullet trap (with a sheet of photographic film in the middle), the bullet may already be in the trap before the shockwave is detected.