Robby, there should be an MR556 at SHOT. I'm planning on attending the live fire demonstration, and HK will likely have one of these available.
There are alot of other [more useful] items making a debut at SHOT this year, though, and the MR556 is about as close to the bottom of my list as you can get.
I was saved from buying a tan frame HK45 only by the fact that HK has not released them yet. Had they been available when I was interested in buying one (about 2 years ago), It would still be sitting in my office while I continue to shoot my 1911 and Glock 19. :lol: