I use a Combro.
There small, cheap and handy. A bit fiddly on an under lever rifle but, it's doable.
I've also used it on break barrels and Co2 rifles.
I've had it for a couple of years now and it's still accurate when compared to results from other chronos.
And, I've never shot it
...... But, I hear Robins customer service is legendary if I ever do.