New York City police have arrested 19 people in what authorities are calling the city's largest-ever guns bust.
Police recovered 254 firearms, Mayor Michael Bloomberg said.
The guns were brought to the city from the US states of North and South Carolina by traffickers travelling on buses, authorities said.
Mr Bloomberg is one of the most prominent proponents of strict gun control in the US, criticising the easy availability of firearms.
"Year after year guns flow into our city from states that don't have common sense gun laws," Mr Bloomberg said on Monday, adding that North and South Carolina were among the top three sources of guns used in crime in the city.
A large majority of the guns were purchased by an undercover police officer in 45 transactions since 2012, authorities said.
The 10-month investigation resulting in the arrests began after police learned that gun sales were taking place in a recording studio in the borough of Brooklyn.
An aspiring rapper had "posted images of guns and cash on Instagram", New York Police Commissioner Raymond Kelly said.
Two accused smugglers, Walter Walker and Earl Campbell, were among those arrested in sweeps that began on 2 August.
Both took multiple trips to New York to sell guns every week, authorities said.
The defendants travelled to the city on discount bus lines that do not require identification for boarding, loading the guns into luggage, police said.
The undercover officer bought all that was offered to him, including parts of an assault rifle, Mr Kelly said.
He "made sure the NYPD was Walker's only customer", Mr Kelly said.
The firearms included high-capacity assault weapons, a fully automatic machine gun and handguns, police said.
The 19 were charged with numerous felony weapons charges.
The police commissioner said that through the course of the investigation, the gun sales took place at locations closer to the bus terminal, where the defendants believed they would be less likely to be intercepted than in Brooklyn.
A wiretap showed the defendants feared becoming ensnared by the police department's stop-and-frisk policy, Mr Kelly said.
A federal judge this month ruled the aggressive street policing tactic unconstitutional and discriminatory. Mr Bloomberg's administration has appealed the ruling.