For those that shoot Rimfire rifle, what do you have to do to ensure the shots register on a shot timer? I have had multiple runs that it didn’t pick up my splits at all.
Standard M&P 15-22 with 40 grn ammo coming in above 1200 FPS. Do i need hotter ammo or a muzzle device? Or is it something to do with the settings on the timer? Are there specific model timers that work better for rimfire than others?

Buy an AMG Lab timer , they can pick up everything everytime.

Guy with the timer should stand on the ejection port side and face the mic thing toward you.

I have a Comeptition Electronics timer and a PACT something. Lots of timers have an adjustable sensitivity as well.

Can’t recall any issue with the Rimfire folks

Same, I have the PACT timer I think it is that LaRue used to sell. The gun club I belong to also has one, to my knowledge, I've never seen issues with .22 as long as they aren't suppressed Subs...

I run a M&P 15-22 without a muzzle device (Ultra Light Weight barrel, basically a 4.5" pistol barrel with a sleeve welded to it to 16") and have no problems with our club's AMGs or my PACT.

Sensitivity setting on the timer would be where I'd adjust.

It can also be an issue of where the timer is physically located.  Are you mounting or holding them?  When held, the recommendation is for the RO to pivot, after the start beep, to hold the timer below the shooter's elbow.

I have a loudener on my 15-22. Its a Tandem Kross compensator. Maybe the game changer one. I havent had any real problem. Our ROs tend to put the timer all up close with RFRI/RFRO shooters