The Detroit Free Press: Officers seize .50-caliber machine gun at Detroit-Windsor border
From the picture it looks like a beautiful example of a .50 BMG, too.
Quick hint to the importer and exporter, next time try "machine tools" instead of "gun parts" when you're trying to fast-track a .50 BMG through customs instead of doing all that annoying paperwork. Or do it legally next time and prevent the whole hassle.
Note well that it was coming from Canada to the US.
And you thought Canadians weren't both crafty and well-armed.