Quoted:
Yeah this should be in the tech forum, but whatever.

A few years ago when I had a gen 3 19, I wanted to replace the serrated trigger with a smooth trigger. My boss at the time had a lot of spare parts and told me I could take a trigger. I later found out they were gen 1 G17 triggers. Basic function check seemed fine, so I took it to the range. I got a couple of inexplicable failures to feed at first, but obviously that had nothing to do with the trigger. But when it began to run right, it immediately went full auto.

Now, I don't know if I was inadvertently bump-firing it, but it sure felt like full auto. I immediately swapped the factory G19 trigger back in, and it went back to normal function. I later learned the cut of the trigger bar on the gen 1 was different than all since. But can someone explain to me why it made my G19 go full auto?

Quoted:
The back of the cruciform on the trigger bar is what catches the front of the firing pin as the slide closes. If the trigger bar and firing pin have too little engagement (an orange partial slide cover plate is made to check this) the firing pin can slip off the cruciform. I suppose under certain circumstances this could cause the gun to go full-auto.