HARRISBURG — Businesses that manufacture frames that can be built into working firearms sued Pennsylvania’s attorney general on Friday, five days after he issued a legal opinion classifying the products as guns under state law.

The Commonwealth Court lawsuit asks a state judge to stop the state police from implementing any new policy, including background checks, based on the written opinion the agency received Monday from Democratic Attorney General Josh Shapiro.

Shapiro told state police to treat unassembled “ghost guns,” gun frames also referred to as 80% receivers, as firearms. The plaintiffs said the opinion does not give fair notice to people regarding what is legal and what is not, said Joshua Prince, who filed the petition.

“What they are saying is, a hunk of metal, because it could be turned into a receiver, is a firearm,” Prince said. “There is absolutely no criteria provided.”

Shapiro welcomed the legal battle, tweeting from his official account: “Bring It! We’ll see you in court.”

“These powerful special interest groups are the same ones that are stunting real reform in Washington and Harrisburg,” he said in a statement. “I will continue to fight to ensure our legal opinion stands and to close this gaping loophole in the system that allows criminals to get their hands on untraceable, unserialized DIY firearms.”