[tomatotomato37]

I think the reason for going after lowers is because that's where, in AR pattern at least, the majority of components for auto-firing are mounted, whereas the upper may just have an extra notch or two etched in the bolt.

[rtkwe]

No it's because the lower is legally the gun, because it has the trigger and hammer assembly in it. The whole rest of the upper half of the gun can be ordered online and shipped right to your house because it isn't legally a gun or the critical components.

[6nf]

Very different in Australia where every part of the gun is restricted. You can’t buy a single gun part and you certainly can’t buy ammunition without a license.

[AWildC182]

The auto sear (auto-firing part of the lower) is just a little lever hinged on a pin that blocks the hammer from moving momentarily. The two cuts that it interfaces with in the upper are somewhat more complicated.

[tomatotomato37]

I thought it also required some sort of twisted wire pin that interfaces with the sear and hinges off something built into the upper.

The majority of my firearm knowledge comes from that world of guns puzzle game so I'm probably very off on something, but I do remember there was a lot of going on in that area of the gun

[dsfyu404ed]

Maybe you're confusing it with the fact that you can make a ghetto drop in auto sear out of a bent coat hanger?

Google "drop in auto sear" eventually you'll find something that explains the sequence of events of the trigger mechanism in sufficient detail.