For the most part, the circuitry either works or it doesn't. If it's not working, you don't get a reading.
For conventional chronographs, there is one condition that will definitely cause an error, and that is lighting. If you are using your diffuser screens and a cloud comes by, your readings will shift. IIRC, it's about 25 FPS with a rifle cartridge.
To do good chronographing, you have to control the lighting, and you have to stick a thermocouple to the barrel just forward of the receiver, and control barrel temperature. If you let the ammunition sit in the chamber for about a minute, the ammunition will effectively come to barrel temperature, so you've controlled both ammunition and chamber.
Those are conditions external to the chronograph.
For the morbidly curious:
The passage of the bullet makes a little dip in the light that the photocell receives. This signal then passes to a Schmidtt trigger, that provides a clean starting edge for the counter. When the counter gets that clean starting edge, it starts counting cycles of the crystal oscillator. When the second sensor gets its edge, the counting stops. The Shooting Chrony oscillator runs at 12 MHz, so if the counter gets 4,000 counts, 1/3 millisecond has elapsed from start to stop. The sensors are 1 foot apart, so the speed is 1 foot / .33333 milliseconds, or 3,000 feet per second, if I've done my math right.
The system is pretty bullet proof. Alas, the chronograph, not so much.
Last edited by denton; 05/22/22.
