1. I can't own ANY such arms if they are modern (made after 1986).
2. If they were made before then, I MAY be able to buy a one (probably worn and with a kludge aftermarket system) for 10x what it would cost if newly made.
Well, I am going by Matt Bracken's "Enemies Foreign and Domestic." In one part, one of the President's advisor's tells him that civilians can own automatic weapons and silencers so long as they pay a $200 dollar tax on them.