In 1927, Congress passed a law barring the USPS from mailing concealable firearms unless they were from licensed dealers in an effort to curb crime. In January, the Department of Justice revisited the 1927 law, calling it unconstitutional and arguing that it violated the Second Amendment, and urged the postal service to change its regulations.

The Justice Department said that as long as Congress chooses to run a parcel service, "the Second Amendment precludes it from refusing to ship constitutionally protected firearms to and from law-abiding citizens, even if they are not licensed manufacturers or dealers."

Last month, USPS proposed a new rule that would allow anyone to mail concealable firearms like pistols and revolvers. USPS currently allows some firearms like long-barreled rifles and shotguns to be mailed, however they must be unloaded and securely packaged. Similar protections would be in place for handguns, which have evolved since 1927. 

In a statement provided to CBS News Thursday, USPS spokesperson David Walton said that rule was proposed on April 2 "to conform with the opinion of the Department of Justice's Office of Legal Counsel regarding the constitutionality of Section 1715 of title 18 U.S. Code, which prohibits the mailing of concealable firearms. The public comment period ended close of business May 4th, and we are reviewing comments."

Nevada Attorney General Aaron Ford, a Democrat who is running for governor, said the rule change would undo the work states like Nevada have done to curb gun violence. Nevada experienced the deadliest mass shooting in modern U.S. history, when a gunman on Oct. 1, 2017, opened fire from the Mandalay Bay casino hotel in Las Vegas, killing 60 people. Following the shooting, Nevada passed a law requiring state-administered background checks on most private gun sales or transfers.

"Our state has suffered enough, and to suggest we make it easier for criminals and abusers to access firearms is a slap in the face to gun violence survivors and law enforcement," Ford said in a statement.

Under the proposed rules, someone could sell and ship a gun to a person within state lines. The rules are tighter for mailing guns across state lines — people could only mail it to themselves in the care of another person and would be required to open it themselves. That's designed to assist people who are traveling to another state where they might want to use a gun for recreation.

The Justice Department argues the patchwork of state laws around guns makes it difficult to take them across state lines for lawful purposes like target shooting, hunting and self-defense. It said that in many cases, people have no ability to travel with a firearm, making mail the "only viable method of transportation."

Ford and other attorneys general in around two dozen states sent a letter Monday urging the USPS to withdraw the proposed rule, saying it will make it easier for people who can't legally possess guns, like people convicted of felonies or domestic violence, to access them. They also said it'll make it more difficult to solve gun crimes. They said the executive branch does not have the authority to ignore a law Congress passed and the rule will override state gun laws.