The devices turn semiautomatic pistols into illegal machine guns, and their appearance in the U.S. has spiked since 2016, according to government data.
Smugglers who run weapons across the southern border are loyal customers.
A study of Boston Police investigative files found that gunshot wounds from medium- to large-caliber rounds were significantly more likely to kill victims than wounds from smaller caliber bullets.
The case is the latest instance in which mainstream digital platforms have been used to facilitate illegal firearms sales.
Fixated on Latin American gangs like MS-13, the Trump administration tiptoes around a major source of their weapons.
Tens of thousands of firearms smuggled from the United States help to fuel extreme rates of violence south of the border.