An unknown number of guns are being built with the mechanisms, causing problems for the police and ATF officials. Photo: An ATF agent holds a generic unfinished receiver, back, and one that has been manufactured, front, at an ATF field office in Washington, on May 06, 2014. The ATF is trying to crack down on the trade in the makeshift guns by targeting shops and individuals who offer to turn the unfinished receivers into functional pieces for firearms. (Photo by Bill O'Leary/The Washington Post)

The best man in a wedding party, who all declined to be identified, stands near the bride and groom as he holds an AR-10 rifle he was handed while the party was having their pre-wedding portraits taken on the steps of the capitol before a rally nearby by gun-rights advocates to protest a new expanded gun background check law in Washington state Saturday, December 13, 2014, in Olympia, Wash. The wedding party was not part of the protest, but posed for pictures with it after being handed it by gun activist Brandon Lyons, who said “we've all just broken the law”, by handing the gun over. Saturday's protest was called the “I Will Not Comply” rally, and those attending said they will openly exchange firearms in opposition to the state's new voter-approved universal background check law, Initiative 594. The law, which took effect on December 4, requires background checks on all sales and transfers, including private transactions and many loans and gifts. (Photo by Elaine Thompson/AP Photo)

Stuart Craig of Washington tries out a Sig Sauer 1911 model Scorpion pistol at the Sig Sauer booth at the National Shooting Sports Foundation's 34th annual Shooting, Hunting, Outdoor Trade (SHOT) Show at the Sands Expo and Convention Center January 17, 2012 in Las Vegas, Nevada. The SHOT Show is the largest annual gathering of shooting professionals with more than 1,600 exhibitors and 30,000 attendees expected. (Photo by Ethan Miller/Getty Images)

A car enthusiast races at the Goodwood Revival historic motor racing festival in Goodwood, near Chichester in south England, Britain, September 11, 2015. The three-day event held each September at Goodwood Circuit since 1998 attracts thousands dressed in period costume who watch motor racing in vehicles from the mid-twentieth century heyday of the venue. (Photo by Toby Melville/Reuters)

A photographer takes picture of an automotive model during the 36th Bangkok International Motor Show in Bangkok, Thailand, March 24, 2015. No automotive event is complete without the latest offering of high-end cars and motorcycles, and the obligatory accessories stationed at each booth: nubile, young auto-show girls in skimpy, skinsqueezing outfits. This year's Bangkok International Motor Show did not disappoint, attracting a larger horde of eager, flesh-seeking photographers than actual fans of automobiles. (Photo by Jurnasyanto Sukarno/JG Photo)

Lady officers of the Philippine National Police force show the taped muzzle of their guns at a police camp in Taguig City, south of Manila, Philippines, 22 December 2014. Law enforcement units all over the country held a ceremonial taping of firearms as a counter measure against cases of indiscriminate firing and illegal discharge of service weapons during the upcoming Christmas and New Year celebrations. (Photo by Francis R. Malasig/EPA)

Bonhams employee Craig Binns polishes a, 1903 two seater moter car, the world oldest surviving Vauxhall, on November 1, 2012 in London, England. The Car is part of a Veteran Car Sale at Bonhams and is valued at around 80,000 pounds (Photo by Bethany Clarke)