Whatever the ATF decides. Just because it has a serial number does not mean that part is considered "the firearm". For pistols the frame is the "firearm" except in those cases such as the P320 in which the inner chassis containing the FCG is the "firearm". AR15 lowers are the "firearm" yet in the case of G3/HK91 clones the removable grip frame with FCG is not the firearm but the rest of the receiver is.So which part is the actual "firearm"?
It varies all over the place more or less at the powers that be's whim.