“There will be lawsuits because it looks like the agency has bowed to political pressure,” said Adam Winkler, a constitutional law professor at the University of California, Los Angeles.
The A.T.F. and Justice Department declined to comment. A senior administration official defended the process and said that the White House had consistently coordinated with the Justice Department. In the weeks leading up to Mr. Trump’s first calls to ban bump stocks, the official said, the White House knew the bureau was open to revisiting a ban.
Yet in tweets, public remarks and statements, Mr. Trump upended the time-consuming, often-overlooked federal rule-making process, which requires coordination among multiple agencies. The president dismissed it as “long and complicated,” vowing that bump stocks would be banned and that the legal review was almost finished.
“I expect that these critical regulations will be finalized, Jeff, very soon,” Trump said in the days after the Florida shooting, directly addressing Mr. Sessions while the agency was continuing its review.
As recently as last week, the public positions of the president and the bureau were out of sync. Mr. Trump announced that the “legal papers” to ban bump stocks were nearly completed and said the devices would soon be “gone.” That same day, bureau officials said that the agency was still reviewing the issue, and that no final determination had been made.
Traditionally, the bureau would study existing statutes, decide that the law permits it to regulate bump stocks and then submit a proposed rule to the Justice Department. The deputy attorney general’s office would make modifications before the budget office reviewed and approved the rule change, and it would then be published in the Federal Register.
The regulation of bump stocks has strayed from that pattern. The bureau determined in 2010 that it could not regulate the devices because they could not be defined as machine guns, which are regulated under the National Firearms Act, the law that regulates firearms in the United States. To prohibit bump stocks, the agency had to contradict that earlier position, essentially saying the statute had not changed but could be read differently to include such a ban.
After the Las Vegas shooting, in which 58 concertgoers were killed by a gunman using firearms outfitted with bump stocks, public pressure grew to ban them. Justice Department and A.T.F. officials privately stuck by their earlier conclusion that they could not outlaw bump stocks, but with little appetite to tackle gun policy in Congress, Mr. Sessions asked the A.T.F. to review its determination.
Agencies periodically scrutinize earlier rulings and change interpretations. What set apart the bump stock evaluation, according to former bureau officials, was Mr. Trump’s overt political pressure.
“No disrespect intended, but I’m sure President Trump has no idea what exactly the authority is for A.T.F. or the D.O.J. to ban these things,” said Bradley A. Buckles, who was the head of the bureau from 1999 to 2004. “In all honesty, he doesn’t know what his authority is in that regard.”
And in altering its 2010 interpretation made under former President Barack Obama, the bureau could be subject to lawsuits that tie up a bump stock ban in the courts.
“The Justice Department and the A.T.F. are going to get sued, and Trump is going to get flak,” said Josh Blackman, a professor at South Texas College of Law in Houston. “Obama was correct in his interpretation of the statute. If you want to ban bump stocks, it has to be done through Congress.”
The apparent reversal on bump stocks appears to be more political than legal, said Rick Vasquez, a former A.T.F. official who supervised the bureau’s prior determination on bump stocks.
“They’re basically playing games with the regulations and the law right now just to make a political statement,” he said.
Other presidents have also influenced Justice Department rule making to help their preferred policies come into fruition, then watched as those rules became fodder for lawsuits. Under Mr. Obama, the Justice Department created the legal underpinnings to protect undocumented immigrants who came to the United States as children after Congress failed to address the issue. That program, Deferred Action for Childhood Arrivals, was the subject of multiple lawsuits.
If bump stock bans are approved, as expected, questions remain on how the rule will be enforced, including whether it will apply to purchases before a ban would be instituted.
On Monday, Mr. Trump continued to publicly press for a ban. “Bump Stocks,” he tweeted, “will soon be out.”