That simple chunk of metal has become the epicenter of a gun control firestorm. A lower receiver is the body of the gun that connects its stock, barrel, magazine and other parts. As such, it’s also the rifle’s most regulated element. Mill your own lower receiver at home, however, and you can order the rest of the parts from online gun shops, creating a semi-automatic weapon with no serial number, obtained with no background check, no waiting period or other regulatory hurdles. Some gun control advocates call it a “ghost gun.” Selling that untraceable gun body is illegal, but no law prevents you from making one.