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)

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)

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)

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)

An instructor guides a student from the General Yermolov Cadet School as she aims with a gun during a field exercise outside the south Russian city of Stavropol January 24, 2015. The General Yermolov Cadet School in Stavropol is a state-run institution that teaches military and patriotic classes in addition to a normal syllabus. (Photo by Eduard Korniyenko/Reuters)

Antonio Banderas attends the photocall for “Pain And Glory (Dolor Y Gloria/Douleur Et Glorie)” during the 72nd annual Cannes Film Festival on May 18, 2019 in Cannes, France. (Photo by Pascal Le Segretain/Getty Images)

“These eerily stunning images taken using infrared lighting reveal the landscapes of Iceland in all their natural glory. Andy Lee, 45, uses a special technique which blocks out light from some visible wavelengths and picks up light from others invisible to the naked eye”. – Caters News. Photo: Location: Kirkjufell. These eerily stunning images taken using infrared lighting reveal the landscapes of Iceland in all their natural glory. (Photo by Andy Lee/Caters News)

The Hubble Space Telescope is shown following its release from the space shuttle Discovery Wednesday, February 19, 1997. The Hubble Space Telescope, one of NASA'S crowning glories, marks its 25th anniversary on Friday, April 24, 2015. With more than 1 million observations, including those of the farthest and oldest galaxies ever beholden by humanity, no man-made satellite has touched as many minds or hearts as Hubble. (Photo by AP Photo/NASA)