Ok, indoors. They work better at picking up the gunfire & giving you correct times. I used to shoot an indoor weekly fun match/stage, with a 22 MKII. Never any problems using a timer in there. But as the RO,,,, knowing how many folks shooting competition do use electronic ear muffs, (to be able to hear range command etc,) the beep of the timer is easily heard by most. Some folks with bad hearing,, usually ask me to make sure I put it close to their ears when I push the button.
Using them,, the start signal is a distinctive beep,,, and it keep recording each shot fired. Once a shooter is done, and showed clear,,, I show them their time. Be careful,,, if you accidentally bump the timer against a hard object,,, it will record that as well. Even a spent brass casing can fly & hit the timer & record it as a "shot." When there is doubt,,, if the shooter knows how many shots they fired, you can go through "review" and see any errors.
The clear out completely & start over when you hit the start button.