WASHINGTON, Jan 14, 2023 (BSS/AFP) - US authorities announced Friday they were tightening rules on a relatively new gun threat that has become more notable in recent years: converting pistols into more lethal short-barreled rifles.

The new regulations target the use of attachable shoulder mounts or braces on guns classified as pistols, which are regulated differently from long guns and the separate category of short-barreled rifles, or those with barrels under 16 inches (40.6 centimeters).

Millions of readily convertible "handguns" have been sold in recent years, frequently semi-automatic weapons that, once equipped with a removable shoulder brace and other attachments, are indistinguishable in function from long guns but for their shorter barrels.

The Justice Department and the Bureau of Alcohol, Tobacco and Firearms (ATF), which regulates guns, said that any handgun with a shoulder brace attached would be considered a short-barreled rifle.

"For decades, the federal government has placed stricter regulations on short-barreled rifles than other firearms because short-barreled rifles are accurate -- like rifles -- and concealable and easily maneuvered -- like pistols," the Justice Department said.

The regulations date back to the surge in criminal gangs in the 1920s and 1930s, when the barrels of rifles and shotguns were often cut short to be used in crime.

The Justice Department said that recently gun manufacturers have sought to "circumvent" the regulations by selling braces which attach to pistols and allow the shooter to stabilize the gun against their shoulder.

The department said weapons like this have been used in recent mass shootings, including the nine killed at a Dayton, Ohio bar and 10 killed in a Boulder, Colorado grocery store.

The new classification means more stringent background checks, taxation and registration requirements in the sale of pistols with braces attached.

However, the new rule does not ban the sale of braces separate from the guns.

"Today's action will save lives," President Joe Biden said in a statement.

The rules will "make it harder for individuals intending to inflict carnage to obtain these weapons," he said.