I taught this in the army. It’s not as difficult if you get them used to pointing with a finger and use middle finger for a bit. After they start getting hang of it, move back to the trigger finger.
It’s a natural instinct that has to be trained if you want to be fast. It has to be natural and done without really thinking. Difficult to teach.
Just curious, did you see any difference in training time between city folk and country folk?