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)

Athletes attend training session at the Ak Bulak Nordic Arena outside Almaty, Kazakhstan, July 16, 2015. Kazakhstan is aspiring to host the 2022 Winter Olympics but many in the Central Asian nation view the bid as yet another vanity project of long-ruling President Nursultan Nazarbayev. Almaty, the financial capital, will go head-to-head with the Chinese metropolis Beijing on Friday when the International Olympic Committee elects the winner at its session in Malaysia. (Photo by Shamil Zhumatov/Reuters)

Tourists visit the Corral Canyon Cave in Malibu, Calif., Friday, May, 6, 2016. The cave, better known by the misleading moniker “Jim Morrison Cave” is now closed to the public until further notice. Large crowds have shown up on a daily basis to see the often vandalized cave and in some cases add to the vandalism with graffiti of their own. (Photo by Damian Dovarganes/AP Photo)

In this April 23, 2015 photo, friends Jia Haixia, left, and Jia Wenqi work to plant a tree in a field in Yeli village near Shijiazhuang city in northern China's Hebei province. For the past 13 years, Jia Wenqi, who has no arms, and Jia Haixia, who is blind, have worked together to plant and water more than 12,000 trees near their village. (Photo by Helene Franchineau/AP Photo)

Kacey Musgraves performs at the 49th annual CMA Awards at the Bridgestone Arena on Wednesday, November 4, 2015, in Nashville, Tenn. (Photo by Chris Pizzello/Invision/AP Photo)

Model Christine Teigen attends the 2014 Billboard Music Awards at the MGM Grand Garden Arena on May 18, 2014 in Las Vegas, Nevada. (Photo by Kevin Mazur/Billboard Awards 2014/WireImage)

A general view of the Vintage and Rare Guitars shop on Denmark Street on April 20, 2011 in London, England. Denmark Street, in the Soho area of central London, traces its musical connections back to the 1890s and is famous for its connection to contemporary British music after recording studios started setting up there in the 1960s. Many world-renowned recording artists, including: Jimi Hendrix, The s*x Pistols and Elton John have played and recorded in the street. The Rolling Stones recorded their first album in 1964 at the street's Regent Sounds Studio's. Known as London's 'Tin Pan Alley' Denmark Street is regarded as one of, if not the, best music streets in Europe and contains numerous specialist music shops, with many boasting of a long and prestigious musical history.

A girl plays with colourful papers at the 23rd Sziget (Island) Festival on Shipyard Island in Budapest, Hungary, 13 August 2015. The festival is one of the biggest cultural events of Europe, offering art exhibitions, theatrical and circus performances and music concerts. Almost 200 performers from 47 countries will entertain the hundreds of thousands of visitors from 11 to 18 August. (Photo by Balazs Mohai/EPA)