Okay, I'm going to describe a firearm:
This gun has a separate upper and lower component. The upper contains the barrel, the breech, the locking assembly, and the firing pin assembly. The sights mount on top and are removable. You can mount a red dot on the upper.
The lower component has the grip, the detachable magazine, a safety, and a subassembly with a removable unified fire control group.
Is this gun a Sig P320? Or is it an AR-15 with a CMC trigger?
How do "modular handguns" exist under the current interpretations of our various gun control laws? It's "not-a-gun" because... the "firearm" is a removable trigger assembly?
I guess the better question is... how come a manufacturer hasn't made a "modular AR chassis system" that's basically exactly a CMC trigger with a serial number plate?