Attorneys general of 20 states are fighting back against an "unconstitutional" Biden administration proposal to regulate gun parts.

West Virginia Attorney General Patrick Morrisey and Arizona Attorney General Mark Brnovich led the 20-state coalition in authoring formal comments urging the rejection of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) proposed new rule on firearms parts that followed President Biden's effort to regulate so-called ghost guns.

"Private individuals and businesses have the right to assemble firearms for their own use — a fact borne out in early American history and expressly recognized by the Gun Control Act," Morrisey said in a statement. "The Second Amendment is a core tenet of our Constitution, and this regulation would treat the activity of assembling firearm parts as a problem to be stamped out, rather than a right and tradition to be respected."

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Biden and Attorney General Merrick Garland earlier this year proposed further regulations of ghost guns to stem gun violence in America. Ghost guns are homemade firearms that aren't marked with a commercial serial number because such parts have been exempt from federal laws.  

The Biden administration effort aims to crack down on individuals buying partially-finished frames and receivers, referred to as "80% receivers," without undergoing background checks or recordkeeping practices with traditional commercial gun sales. Everytown for Gun Safety, a gun-control group, has argued that the lack of regulation of ghost guns is the fastest-growing gun safety problem in America. 

But the GOP-led states contend that federal law authorizes the ATF to regulate complete firearms and receivers, not the individual parts of an incomplete receiver, and new regulations could put certain gun parts manufacturers out of business. 

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"By allowing ATF to decide for itself which firearms it will regulate, unconstrained by Congress’s guidance, the proposed rule is unconstitutional," the attorneys general wrote.

In addition to West Virginia and Arizona, the attorneys general of the following states have signed on: Alabama, Arkansas, Alaska, Florida, Georgia, Indiana, Kansas, Kentucky, Mississippi, Missouri, Montana, Nebraska, New Hampshire, North Dakota, Ohio, Oklahoma, South Carolina and South Dakota.

But blue state attorneys generals, however, have urged the Biden administration to further regulate gun parts. In March, Pennsylvania Attorney General Josh Shapiro, a Democrat, led a coalition of 18 states calling on Garland to crackdown on ghost guns that evade background checks.

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"Ghost guns are quickly becoming the weapon of choice for criminals and fueling the gun violence epidemic," Shapiro said at the time. "These DIY gun kits should be subject to the same background checks and qualifications as fully functioning firearms to prevent criminals who are not legally able to purchase or possess guns from getting their hands on these deadly, untraceable weapons."

The public comment period for the proposed gun parts rule closed on Aug. 19. Such comments are designed to guide the ATF in drafting the final regulation.