In USPSA, your gun will be weighed, it has to fit in a box, and the thumb safety has to work. After that, 3 rounds are fired over TWO chronographs, the velocities multiplied by your bullet weight from the round they pull a bullet from, and the power factor is calculated by taking the average and truncating any decimal.
If you don't make power factor, they fire three more rounds and average again.
If you still don't make power factor, the last of the 8 rounds collected from you, you're given an option. They can pull the bullet and re-calculate the power factor based on that bullet's weight or they can fire it and re-average your numbers and see if you make it.
If you don't make major, you're scored minor.
If you go sub minor, you shoot for no score.
If your gun is overweight or doesn't fit in the official dimensioned box, you get bumped to the Open division, or if it's not offered at that match, you shoot for no score.
If your thumb safety doesn't work, that gun is out of the match unless you can fix it. If you have another one and the rangemaster says okay, you shoot that gun. Both the old and new gun have to be chroned if it's safe to fire, but you won't be allowed to holster the gun with the broken thumb safety again.
If you refuse or miss chrono somehow, you shoot for no score.
And perhaps most importantly of all, if the chrono breaks, fails calibration, or is otherwise unavailable, everyone who was chronoed when the equipment was working gets the results they got, and everyone else gets their declared power factor regardless of the fact that they can't chrono. I've never seen or heard of that happening.
The last major match I was at, a guy came all the way from Canada to shoot for no score because his safeties were about .010" too wide to fit in the box.
I also shot one years ago where one guy loaded for the whole squad (posse) and all of them went minor, and it wasn't even close, 150-155 when the pf for major is 165.
And as Bogus said, chrono is a stage. Everyone has to chrono when it's their time at that stage. Usually it goes quickly so it's nice if it's at lunch time, you'll get a longer break before the squad ahead of you gets done shooting. They also don't have to let you use the 8 rounds you gave them, they can ask for a mag off your belt anytime they want.
This is recommended at all state level competitions and required at all regional and national level matches.
This procedure isn't perfect but I've never seen someone get screwed by it. Although I barely made it at my last match--I won't go that close again. I'm not one to fly too close to the sun.