[circlecity]

"The lower receiver is of course a part of the gun that contains the firing mechanism, and is basically what makes the gun “work”."

I'm not sure what this "firing mechanism" is he speaks of unless he's referring to the trigger. Since most of a lower is polymer anyway this would be the easiest, most reliable part to create with a 3D Printer. The bolt, bolt carrier group, action and charging handle are all in the upper, all part of the "firing mechanism" and all take the most stress in firing along with the barrel itself.

[Gandalf_The_Gray]

"The lower receiver is of course a part of the gun that contains the firing mechanism, and is basically what makes the gun “work”."

The part that "makes the gun work" is often referred to as "AMMO", without ammo the empty gun is known as "A CLUB".

Regards,
GtG