The arrest Saturday of three teenagers who allegedly attempted to trade $1,500 worth of crack cocaine for a submachine gun reflects a "flourishing black market" in which high-powered weapons are swapped for drugs, an official of the Bureau of Alcohol, Tobacco and Firearms said yesterday.

David C. Troy, special agent in charge of the bureau's Washington district office, said deals in which drugs are offered for such firearms are "one of the most common ways that guns are traded . . . . I would say there is a flourishing black market for weapons."

The three youths were arrested after they allegedly tried to trade the drugs with an undercover officer in a Northeast Washington parking lot, officials said. They were charged as juveniles with possession of cocaine, and are expected to be arraigned today.

Troy said the investigation began after an informer's tip that the three young men had made it known that they were interested in trading crack for a submachine gun. He identified the three as crack cocaine dealers operating in Northeast.

The official said that after the tip was received, an agent posing as a black-market gun dealer contacted the youths and negotiated the amount of cocaine to be traded for a fully automatic 9mm MAC-10 submachine gun, then set up a place and time for the exchange. Several agents and D.C. police officers from the 5th District were involved in the arrest, which took place about 3 p.m. in a parking lot in the 1500 block of Bladensburg Road NE.

Troy said fully automatic MAC-10s are illegal weapons commonly used by drug dealers. The pistol-sized weapon, which is no longer being manufactured, originally was made as a semiautomatic but can easily be converted to a fully automatic submachine gun, Troy said.