Assembling Automatic Weapons

As a licensed gun dealer, Mel Bernstein can buy, sell and shoot machine guns -- weapons considered so dangerous their sale is carefully controlled and buyers are rigorously screened by the government.

"You have to be super good to get approved for an automatic weapon, you have to have almost a super clean record," he told CBS News Correspondent Cynthia Bowers. "Anything you did in your lifetime can be used against you when you send in your paperwork to Washington.

But a joint investigation by CBS News and Denver affiliate KCNC found that isn't always the case. Just because you're not approved for an automatic weapon, doesn't mean you can't get one.

Anyone who wanted to modify a gun into an automatic weapon could easily find all the parts needed for sale at a recent Colorado gun show. It was the same gun show where Eric Harris and Dylan Klebold bought the weapons used in the Colombine massacre.

And that easy availability has upset a lot of people.

"Post Columbine, we all realized that gun shows were a problem," said Colorado lawmaker Dan Grossman.

So Colorado voters did something about it. They passed tough, new, first of its kind legislation requiring background checks to be run on everyone who buys a gun at a show. But an apparent loophole allows people to buy parts and instructions to turn legal rifles into illegal machine guns -- fully automatic weapons capable of firing up to 600 rounds a minute -- without background checks.

One man, who asked not to be identified, found out the hard way that just because something's for sale doesn't mean it's okay to own it.

"I'm outraged," he told Bowers. "I assumed after Columbine that would be the safest place. Everything for sale would be heavily scrutinized."

So he assumed it was legal to buy the tiny parts needed to make his semi-automatic rifle fully automatic.

And, even though he never installed the parts, just having them and the gun together, was enough to get him a federal firearms conviction. He now faces more than two years in prison.

But it's not the ones they catch that worries law enforcement; it's the ones they don't.

"I think violations do go on at gun shows, but I think that violations go on all over the state -- in homes, cars, in garages," Jerry Petrilli of the Bureau of Alcohol, Tobacco and Firearms. "We can't be everywhere."

And officials are aware of the potential for problems. Consider the deadly shootout in North Hollywood California, when police responding to a bank robbery found themselves outgunned by criminals carrying illegal fully automatic AK-47s.

"We have to recognize the second amendment rights of our citizens but we have to be reasonable," said Grossman. "No one needs to have a machine gun for self protection."

Part One: One-Stop Shopping For Bombmakers