Ghost guns

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2. Loophole:

An ambiguity or inadequacy in a system, such as a law or security, which can be used to circumvent or otherwise avoid the purpose, implied or explicitly stated, of the system.
Since the system, in this case the ATF rules, were designed to allow individuals to make their own guns, this isn't a loophole. Like the "gun show loophole" this rule was intended to allow the kind of activities considered objectionable by the group bringing the lawsuit.

Neither 80% lowers not being considered firearms nor the ability of an owner to privately sell a gun, their personal property, were planned to be regulated by federal government. If one is a prohibited person such as a felon, domestic abuser or has been dishonorably discharged from the military, it is still illegal for them to manufacture or possess a firearm.

IMO, laws best serve as criteria for juries and courts to use to determine the guilt of the accused. Laws that are aimed at enforcing compliance by the citizenry to standards of behavior held by the legislature (or actually only by a majority thereof) and enforced by an, at times, hostile police officer or unit are less than ideal.

It is undesirable to have government in the role of controlling the behavior of private citizens. Laws intended to provide such controls are often impossible to enforce and frequently lead to bigger problems.