> You do not print a lower receiver compatible with standard parts.

I've personally seen 3d-printed polymer Glock frames and AR-15 lowers. I've personally seen them stand up to sustained automatic fire. You're not going to be printing with PLA and there's plenty of bad designs out there that can't take the stress, but good designs that are durable when printed with the appropriate filament or resin exist.

> Printing plastic has a very hard time confining the gunpowder.

Which is why printing Glock frames and AR-15 lowers works so well—they're the controlled part and they don't bear the pressure of the gunpowder combustion products.

> Printed guns are ghost guns, but the vast majority of ghost guns are not printed guns

Sure, you're probably seeing way more Polymer80 and aluminum 80% lowers than 3d printed ghost guns... but people aren't CNCing those at home, either. They were milling those out with special jigs and a drill press.