btw there are 2 ways you could attempt to detect the bullet:
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By illuminating the area with steady IR and looking for a fairly sharp change in the received IR.
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By modulating the IR and using a synchronous detector to look for received IR at the modulation frequency. Since the bullet will only be over the sensor for about 20us, you would need to use quite a high modulation frequency, say 500KHz (which is about the maximum possible, given the 800nS rise/fall time of the emitter). This approach should be less sensitive to muzzle flash.