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)

A Panasonic Toughpad is displayed during a press event at The Venetian for the 2012 International Consumer Electronics Show (CES) January 9, 2012 in Las Vegas, Nevada. CES, the world's largest annual consumer technology trade show, runs from January 10-13 and is expected to feature 2,700 exhibitors showing off their latest products and services to about 140,000 attendees. (Photo by Ethan Miller/Getty Images)

An employee looks at the artwork “4-Chlorephenol, 2008”, part of the artist Damien Hirst's exhibition “The Complete Spot Paintings” at the Gagosian Gallery on January 12, 2012 in London, England. (Photo by Matthew Lloyd/Getty Images)

An authenticated contemporary copy of Leonardo da Vinci's Mona Lisa on display at the Prado Museum on February 1, 2012 in Madrid, Spain. Experts of the Prado Museum say the copy was painted at the same time as the Mona Lisa in Leonardo Da Vinci's studio by one of his students.

An Egyptian actress working at the Pharaonic Village simulates ancient agriculture scenes during a show marking Sham el-Nessim, or “smelling the breeze”, in Giza, Egypt, Monday, April 13, 2015. The holiday signifies the arrival of Spring, a uniquely Egyptian tradition practiced since the days of the Pharaohs. (Photo by Amr Nabil/AP Photo)

A Zimbabwean subsistence farmer holds a stunted maize cob in his field outside Harare, January 20, 2016. About 14 million people face hunger in Southern Africa because of a drought that has been exacerbated by an El Nino weather pattern, the United Nations World Food Programme (WFP) said on Monday. In Zimbabwe, 1.5 million people, more than 10 percent of the population, face hunger, WFP said. (Photo by Philimon Bulawayo/Reuters)

A reveller takes part in the battle of “Enfarinats”, a flour fight in celebration of the Els Enfarinats festival on December 28, 2015 in Ibi, Spain. Citizens of Ibi annually celebrate the festival with a battle using flour, eggs and firecrackers. (Photo by Pablo Blazquez Dominguez/Getty Images)