Hall effect sensors to detect high velocity projectiles

d-tinkerer:
@sjarls did you finally get to building your DIY chrony? I am thinking of doing the same. Assumed it would not be too difficult. But looks like hall effect sensor might not work? How about ultrasonic sensors?

d-tinkerer,
Don't hold your breath waiting for a reply from sjarls. He made 5 posts back in 2017 (the last 2 being in this thread) and hasn't been heard from since.

Assumed it would not be too difficult.

If it was not difficult, then they would be on sale for a few £/€/$.

How about ultrasonic sensors?

Another non-starter:

Using sound to measure something that could be supersonic isn't going to work.