Correct: the gas block, barrel, breech, etc., are all "upper" bits and take most of the physical stress. The lower includes the trigger assembly and magazine h

Correct: the gas block, barrel, breech, etc., are all "upper" bits and take most of the physical stress. The lower includes the trigger assembly and magazine holder thingy, which are the serialized, "this is a gun" parts. Conceivably one could 3D-print the lower receiver, order the upper assembly, rotating bolt, recoil buffer, etc., through the mail with a prepaid card, and have an essentially untraceable rifle.