Using an interrupt since if im not mistaken, thats the fastest sensing capability of the arduino is it not? And yeah 545km/hr is fairly fast. THats a pertty standard outdoor projectile speed though. Many guns however fall into the 350fps range (~380km/hr for AWOL).
It probably is however the BB is too small to block out the light enough for the sensor. Though I know when I tried to do this on a couple pistols I have, it sensed just fine. So probably speed coupled with the small cross section.
So then, what could one do then to make the sensor read better, cause I cant really change the size of the BBs in the gun.
TY btw