What part of an AR is considered the "firearm"

[9orangeletters]

I've been wracking my brain and Google over this today and so far I'm more confused that I was when I started.

Simply put, what part if the AR is considered the firearm, that the purchasing of would trigger a PICS/NICS check to purchase?

[PaBimmerGuy]

the part with the serial number which is the lower receiver. doesnt matter if it has any parts installed. the lower is the gun.

[ByAnyMeans]

[Noah_Zark]

the part with the serial number which is the lower receiver. doesnt matter if it has any parts installed. the lower is the gun.

What PBG said.

In its basic stripped form, the typical AR lower receiver looks like this:



Noah