In Seattle at 600 feet. the problem is pupil size with dark cloudy skies, and I have to turn down the power on the scope.
But in Montana at 2600 feet, the sky is black when looking straight up. THere is no background for the chrono to see the bullet.
But if I tip the chrono sideways, so the background is a white storage box, the the chrono triggers every time.
Of course, when there are clouds at 2600 feet, the box is not needed.
I made it through this post without mentioning Einstein and Oxygen scatting blue light. [why you can look at a sunset]