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.