An investigation by the network, which included gun purchases recorded by hidden cameras, found hundreds of thousands of guns for sale on hundreds of Web sites.
Steve Barborini, a former supervisor for the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives hired by the network to help with the investigation, said online sales create "a weapons bazaar for criminals."
"There's no background check: Anybody that has a murder conviction can simply log on, e-mail someone, meet them in a parking lot and buy a freaking AK-47," Barborini said.
For the report, NBC News hired two Arizona security experts who posed as a husband and wife seeking to buy guns listed in online ads for sale in Arizona, one of many states where such sales are legal.
Within 12 hours, undercover buyers had purchased eight guns, including a .50-caliber sniper rifle powerful enough to bring down a helicopter.
Unlike at stores, no background checks were required for the online buys.
A bill sponsored by U.S. Sen. Charles Schumer, D-N.Y., would require background checks for all guns sales, including those online, but the bill has been stuck in committee nearly a year.