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)

Jeison Rodriguez (L), 19, the living person with the largest feet in the world, poses for a picture with his older brother Wilmer, 24, outside their house in Maracay, Venezuela, October 14, 2015. Rodriguez, holds the Guinness World Record for the largest feet, with 40.1 cm (1 ft 3.79 in) on the right foot and 39.6 cm (1 ft 3.59 in) on the left foot. (Photo by Carlos Garcia Rawlins/Reuters)

Actors Ma Sichun (L) and Zhou Dongyu pose backstage after both won the Best Leading Actress award for their movie “Soul Mate” at the 53rd Golden Horse Film Awards in Taipei, Taiwan November 26, 2016. (Photo by Tyrone Siu/Reuters)

A couple kisses during a LGBT Pride parade in Oviedo, northern Spain, June 28, 2016. (Photo by Eloy Alonso/Reuters)

People walk through crop circles in a cornfield near Raisting, Germany, on July 28, 2014. According to media reports, a balloonist had discovered the circle some days ago. Since then, hundreds of people came to the field to watch it, however it is unclear who did create the pattern. (Photo by Karl-Josef Hildenbrand/DPA)

Businesswoman and vendor Sonia Maria Paypay sells clothing at a stand at the commercial zone of Gamarra in Lima, April 16, 2015. (Photo by Mariana Bazo/Reuters)

A guard dog in the suburbs of Yakutsk, Russia, considered to be the coldest city in the world, January 2014. (Photo by Amos Chapple/REX Features)

People take photographs of waves crashing against the harbour wall on January 3, 2012 in Porthcawl, Wales. (Photo by Matt Cardy/Getty Images)