Back in the early 80s a cottage industry sprang up where small shops would import non transferable NFA items like WW2 surplus, make a new registered receiver, strip off all the parts and assemble on the new receiver. This was great for a while as it reduced the cost of owning a lot of machine guns that otherwise were difficult to obtain. Some shops were better than others, Doug Offinger's Browning belt fed guns were just as good as the original ones while some shops producing Stens were just selling junk.