I'm interested what practices were taught or developed to avoid pulling the trigger with no cartridge in the chamber?

Apparently incorporating a slide lock follower in the magazines wasn't considered worth the drawbacks when the carbine was only a semi-automatic.
Now I know plenty of other firearms have no 'out of ammo' feature including revolvers and bolt action rifles used by the militaries. But I'm curious as to what was taught and what guys actually did so as not to hear a 'click' in a critical situation.
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