Americans can turn a handgun into a machine gun with a cheap piece of plastic.
As someone who can count the number of guns I've ever seen on my hands, this is incredibly sensationalist. Machine guns are rifle-calibre automatic weapons with significant weight and recoil. You aren't making one out of a handgun and some cheap piece of plastic if you are not living in Naked Gun.
As a other commenter mentioned machinegun is a legal definition in the US, for a firearm capable of automatic or burst fire.
Here the author is referring to Glock switches an aftermarket design that exploits the design of semiautomatic Glock pistols to convert them to be automatic.
My understanding is that typically most of them tend to be ones bought online and shipped from China in bulk then resold once in the states.