How easy is it to get an assault weapon?

judge (woman): secret? this morning on rossen reports, gun violence back in the news with the massacre in aurora. turns out colorado has some of the weakest in the country. how easy to get a weapon. good morning. how are you?

>> good morning. turns out he had no rap sheet. he was able to buy his guns in a store legally. even if he had a violent criminal record he still could have bought them. this morning we're going to show you how easy it is for anyone, even criminals, to buy assault weapons . inside this theater police say james holmes opened fire with a stash of guns like these. the same kinds of guns we were able to buy. no background check , no questions asked. it's this easy. hundreds of thousands of guns for sale on hundreds of websites. we responded to gun ads and set up meetings at popular shopping malls .

>> how much was it we agreed on?

>> 500.

>> we bought the handgun similar to the one holmes used.

>> he could be a dangerous criminal although you still sold him the gun with hollow point bullets.

>> absolutely.

>> assault rifle . holmes used one of these.

>> you sold the man a dangerous weapon . what if he went and shot somebody with it.

>> remember, at gun stores background checks required but here nothing. james holmes bought 6,000 rounds of ammo online. in most states like colorado it's completely legal. earlier this year we went online in phoenix, arizona, another state where it's legal. within minutes we had meetings set up. our gun buyers, two security experts we hired posing as husband and wife.

>> how are you doing?

>> our hidden cameras are rolling as this man pedals a glock. we even hinted we were criminals.

>> no paperwork, background check because i probably wouldn't pass one anyway.

>> really.

>> anyway, man, i appreciate everything.

>> they shake on it.

>> jeff rossen with nbc news. he told you he couldn't pass a background check . did that raise any red flags .

>> slightly. when you need the money you need the money.

>> people can get hurt. does that matter?

>> yes.

>> didn't matter to you?

>> i need the money.

>> we could hardly believe it but twists and turns just beginning. this seller shows up with a tactical shotgun, assault rifle and 7-year-old son.

>> i appreciate that. no background check because i probably couldn't pass anyway.

>> watch as he hands the cash to his boy.

>> jeff rossen , nbc news.

>> not interested in talking to you.

>> he told you he couldn't pass a background check and you still sold him the weapons. the most scary transaction of all came after dark in this pharmacy parking lot. the online ad for this 50 caliber sniper rifle , the most powerful gun legally sold in the u.s. it can pierce armored vehicles , even bring down a helicopter. but the seller is so laid back, you'd think he was hawking a used bicycle.

>> enjoy. i know you're going to like it.

>> once he got the cash, the gun was ours, no questions asked. we had some for him.

>> this is the drug of choice for drug cartels . this can take down a helicopter.

>> it can take down anything, 50 caliber .

>> you know how powerful it is. doesn't it concern you you have no idea who they are. they could be terrorists?

>> no.

>> doesn't concern you?

>> no.

>> as the country mourns this tragedy in aurora, the country mourns again.

>> the reason it happens over and over , it can be traced to weakness in our gun laws . this doesn't have to happen.

>> there is legislation that would make it illegal to buy a gun anywhere without a background check , but it's been sitting in congress for over a year. the nra opposes the bill, savannah, saying that bill has many serious flaws.

>> jeff rossen , thanks for the reporting. appreciate it.