The woman caller dialed the radio swap show with something to sell.

It was blue steel, 3 1/2 feet long, and fired a thousand rounds a minute.

Just your average housewife selling a rare machine gun.

"Lots of weird things have been sold on this show, [but] when she said, 'I have a machine gun,' I was a little taken aback," said Ned Richardson, host of the "Trading Post" show in Danville, Va., for nearly 25 years.

But this time there wasn't a successful sale. Instead, federal agents, tipped off by the call to the show, seized the machine gun. It turned out to a rare, pre-World War II German aircraft machine gun worth about $5,000 that had been stolen from the Smithsonian Institution.

Now, federal agents are trying to figure out how the machine gun ended up in Danville. The identity of the woman who called the radio station has not been disclosed. Nolan Douglas, an official with the Bureau of Alcohol, Tobacco and Firearms, said that no one has been arrested, but that if anyone is, he or she could be charged with possession of an unregistered machine gun, an offense that carries a maximum penalty of 10 years in prison and a $10,000 fine.

Museum and federal officials declined to provide specific details about the theft and recovery of the weapon, saying that the investigation is continuing. Douglas, the agent in charge of ATF's Richmond office, said there are two suspects in the case.

Over the years, Richardson's offbeat call-in show has sold almost every imaginable item -- from a post office building to a boa constrictor. One woman even found a child to adopt.

"It's sort of like a family reunion every day," said Richardson, whose folksy personality has made him a Danville institution on station WBTM ("World's Best Tobacco Market").

But the machine gun offer was a first for the show.

"I told her, 'Lady, you're violating the law if you have a machine gun . . . unless it's filled with lead.'

"She laughed like I was joking."

But local and federal authorities didn't laugh.

They called the station almost immediately and got the woman's phone number, Richardson said.

The air-cooled, MG-15 was reported missing when it disappeared last August from the Smithsonian's restoration center in Prince George's County where it was being readied for an exhibit at the National Air and Space Museum, according to museum security official James F. McGuire. It fires 7.9-mm. bullets and weighs nearly 30 pounds.

There have been at least three other major thefts from the Smithsonian in the last two years. In March, two intricately decorated swords and two gold medals were stolen. In February, an ornate silver pen used by statesman John Hay to sign the Treaty of Paris in 1898 was stolen from the Smithsonian's Museum of American History, and a $125,000 snuff box disappeared from the National Collection of Fine Arts in September 1979.

Back in Danville, Richardson, 54, still marvels at the woman with the machine gun. He said local rumor has it that the woman got the weapon from two men who asked her to get rid of it for them.

In one sense, it was logical for her to call his show, which has a reputation for moving items that are hard to sell. And in the show's fine tradition, the woman's description of the machine gun turned out to be reliable.

When federal agents seized it, they discovered that it still could be fired -- just like the woman said.