For Brian Pfleuger, Gura thinks the common use test is the primary contribution (outside of the ruling itself) from the Heller case, and further, that "in common use" actually means, "in common use, or would be in common use".
He was addressing a question along exactly the lines of your "concern", where, if a weapon is banned and vanished from the earth, wouldn't it then automatically fail the common use test? He said no, and then explained it with the "would be" verbiage, adding that "would be" is how a newly-invented weapon could also pass the common use test.
I don't know what Gura says has much bearing, the real question is WWSD?
What would Scalia do? His explanations on NFA items run counter to the above arguements and he's continually shown a desire to reserve some rights on restrictions for the court.