Your only hope of legally owning one is to find one of the VERY few that made it into the US before the 86 ban. If you do its gonna cost you a LOT!! i heard awhile back that one of them went for just under 40k at auction.
Your other option is to become a class 3/NFA dealer and get an LEO demo letter from your local PD/Sheriff/State patrol. My buddy has one and he has all kinds of fully automatic weapons i.e. HK G-36 series, FN P90's, MP5's, M4's of all calibers, a converted Glock 17 full auto etc etc etc
I have been looking into getting mine, I just need to jump through some hoops and spend the money to do it