I have one. It works OK. Good under florescent lighting, that's why I bought it.
Reads in meters per second.
Think it cost £17 from bang good. If you join you can get a discount and they do promote black Friday deals.
I think it's called a velocity meter.
https://m.banggood.com/Muzzle-Speed-...r_warehouse=CN
This one has a tripod. Install the app and get more discount.
My chronograph is designed to clamp on the end of a barrel but I just shoot through it.