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)

Maj. Matt O'Donnell of Glenelg, MD shields himself from rotor wash as Ospreys carrying the delegation of U.S. Secretary of Defense Leon Panetta leaves on March 14, 2012 at Forward Operating Base Shukvani, Afghanistan. Panetta is also scheduled to meet with President Karzai during his two-day visit to Afghanistan. The trip comes as the Taliban has vowed revenge following this past weekend's killing spree by a U.S. soldier who is accused of killing 16 civilians in Afghanistan. Most of those killed were children and women. (Photo by Scott Olson/Getty Images)

People pose for photographs in the wind during Typhoon Saola in Tseung kwan O, in Hong Kong on Saturday, September 2, 2023. Typhoon Saola made landfall in southern China before dawn Saturday after nearly 900,000 people were moved to safety and most of Hong Kong and other parts of coastal southern China suspended business, transport and classes. (Photo by Billy H.C. Kwok/AP Photo)

Democratic presidential candidate and US Vice President Kamala Harris reacts as she holds a photographer's camera while flying on Marine Two, on her way to O'Hare International Airport, Chicago, Illinois, on August 20, 2024. Harris is traveling to Milwaukee, Wisconsin, to speak at a campaign event. (Photo by Kevin Lamarque/Pool via AFP Photo)

Pro-government fighters stand next to a tank destroyed during recent fighting in Yemen's southwestern city of Taiz March 14, 2016. (Photo by Anees Mahyoub/Reuters)

Competitors in the Pro Bikini division pose during the Arnold Classic Australia at The Melbourne Convention and Exhibition Centre on March 14, 2015 in Melbourne, Australia. (Photo by Robert Cianflone/Getty Images)

Activists dismantle Ukraine's biggest monument to Lenin at a pro-Ukrainian rally in the central square of the eastern city of Kharkiv, Ukraine, Sunday, September 28, 2014. (Photo by Igor Chekachkov/AP Photo)

A pro-independence protester sits in front of Catalonia's regional parliament as lawmakers vote inside, in Barcelona, January 16, 2014. (Photo by Albert Gea/Reuters)