A legal loophole means that anyone, including criminals, can order a so-called "ghost gun" off the web without a background check - a gun with no serial number that can't be traced.
The guns are built from kits and arrive in pieces, so under existing law, when they're shipped, they aren't guns. When assembled by their buyers, they're lethal - and legal.
Federal officials like Graham Barlowe, the resident agent in charge of the ATF's Sacramento office, say the loophole is dangerous.
"People that could not pass a background check," said Barlowe, "are purchasing these unfinished receiver kits and making firearms because they know that if they went to a gun store, they wouldn't be able to pass a background check."
SEE: States with the toughest gun laws:
States with the toughest gun laws
States with the toughest gun laws
National Rifle Association's (NRA) annual meetings and exhibits show in Louisville, Kentucky, May 21, 2016. (Reuters)
#24. West Virginia
Score: 18 Grade: D-
West Virginia may rank low in terms of overall gun control, but in 2015 Democratic Gov. Earl Ray Tomblinvetoed a bill that would have allowed people to carry concealed weapons without a permit.
Score: 19.5 Grade: D-
The relatively lax firearm laws in Indiana have led some to link them to gun violence in Chicago.
#22. New Hampshire
Score: 20.5 Grade: D
New Hampshire requires handgun dealers to obtain a state license, but does not extend the rule to dealers of rifles and shotguns.
Score: 21.5 Grade: D
Nebraska differs from many states in that it gives local jurisdictions some control over firearm regulations.
Score: 22 Grade: D
Following the Virginia Tech shooting in 2007, the state took greater measures to report mental health records and prohibit sales of firearms to any person who has been "adjudicated as a mental defective."
Score: 24 Grade: D
Ohio does not prohibit the sale of assault weapons nor limit the number of firearms that may be purchased at one time.
Score: 25 Grade: D
In a major blow to gun control advocates, Republican Gov. Scott Walkerrepealedthe state's mandatory 48-hour waiting period for handgun purchases in 2015.
While Colorado requires licensed firearms dealers to process background checks, it does not impose limits on the number of firearms that may be purchased at one time.
Score: 30 Grade: C
Michigan gun control laws require firearm owners to report handgun purchases and theft of any firearm.
Score: 31 Grade: C
Oregon greatly strengthened its firearm regulations in 2015 when it enacted laws requiring background checks for all firearm purchases.
Score: 35 Grade: C
Compared to other solidly Democratic states, Pennsylvania's gun control laws are relatively lax, with no requirements for firearm owners to obtain licenses or register their weapons.
Score: 36.5 Grade: C
Although federally licensed firearms dealers must initiate background checks, Minnesota does not require private sellers to do so.
Score: 47.5 Grade: B-
Washington requires all firearm dealers to obtain a state license, but does not mandate that individuals register their weapons.
Score: 50.5 Grade: B
Delaware has relatively strict gun control measures, including mandatory background checks and purchase restrictions on domestic violence misdemeanants.
#9. Rhode Island
Score: 58.5 Grade: B+
Rhode Island may have comprehensive gun control measures overall, but it stilldoes not prohibit the transfer or possession of assault weapons.
Score: 60 Grade: B+
In addition to requiring background checks, Illinois has instituted some design safety standards for handguns.
Score: 73.5 Grade: B+
In 2014, Hawaii had the lowest number of firearm-related deaths per 100K people. Gun regulation advocates have attributed this to its tight gun control measures.
#6. New York
Score: 81 Grade: A-
New York enacted major gun control reforms in 2013, including an expansion of the assault weapons ban and requirements for ammunition dealers to conduct background checks.
Score: 81.5 Grade: A-
Following the New York reforms, Massachusetts passed a comprehensive gun control bill in 2014 that increased firearm regulations.
Score: 82 Grade: A-
Among Maryland's firearm regulations are a ban on the transfer of certain assault weapons and a mandatory seven-day waiting period prior to the physical transfer of a firearm.
#3. New Jersey
Score: 86 Grade: A-
Republican Gov. Chris Christie may have shifted to the right on the issue of gun control, but his state still has some of the tightest firearm regulations in the country.
Score: 86.5 Grade: A-
Connecticut's extensive gun control laws require firearms dealers to obtain a license and mandate background checks for all firearm sales at gun shows.
Score: 93.5 Grade: A-
California takes the top spot as the state with the strictest gun control measures. Among the regulations in place are required background checks and limitations on the number of handguns an individual can purchase each month.
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Police say criminals are well aware of the availability of "ghost guns," and they've been used in shootings across the country, from Maryland to California.
Jeff Rossen, NBC News national investigative correspondent, went online to see how easy it would be to order these gun kits. He quickly found dozens of websites offering the product, and ordered a rifle kit, which he had shipped to former ATF agent Rick Vasquez in Virginia.
All the parts needed to assemble a gun were in the box when it arrived. It took Vasquez a couple of hours to assemble the weapon.
"This is now a completed semi-automatic firearm," said Vasquez, showing it to Rossen. Rossen noted that there was no serial number on the finished product, making it untraceable.
Said Vasquez, "That is correct ... You cannot trace this firearm." He and Rossen then took the weapon to a range and fired it, where in Vasquez's expert opinion, it "work[ed] great."
Sen. Chuck Schumer, D.-N.Y., the senate minority leader said he wants to close the loophole, but expects fierce opposition from the National Rifle Association. "We are going to try to pass legislation," said Schumer. "The trouble is the NRA is so unreasonable and has such power in the Congress, you'd think this should pass like that, but it is going to be a long hard road."
The NRA did not respond to repeated requests for comment.