If you are going to attempt to detect the bullet using reflected IR, here is some advice:
-
You need an IR emitter with high radiant intensity. The output of an IR emitter is usually quoted in mW/Sr where Sr is a measure of solid angle. Narrow beam emitters naturally have higher radiant intensity than wide angle emitters. So go for a narrow beam high intensity IR emitter (assuming the bullet path is predictable) such as http://uk.farnell.com/vishay-semiconductor/tsal5100/ir-emitter-2-6v/dp/1469615, not the sort used for TV remote controls. You may wish to use several IR emitters.
-
For detecting events of this speed, a PIN photodiode is better than a phototransistor. The disadvantage is that it needs more amplification; however the rise and fall times are much faster, around 1us is not hard to obtain.
-
Shield the detector from extraneous IR. However, scattered light from the muzzle flash may be a serious problem, even if the muzzle flash is well out of sight of the detector.