I know this is an awfully old thread, but I know a legal process you can jump through if you want to SBR one.
1) Get your FFL to receive the pistol variant
2) have him then pin&weld an 8" barrel extension. HK Parts sells one
3) have him add a fixed stock.
4) have him remove the 30-round magazines, because you cant own new production ones there
5) purchase the rifle from him!
Now it is legally a rifle, with no adjustable stock, no bayonet lug, no barrel shroud, and no threads at the end of the barrel, so you can legally purchase it in MA. Go down to your PD with the proper fingerprint cards and have then print you, go down to AAA or wherever to get a passport photo taken, and then e-file your Form 1 on the ATF's portal, and by the end of the month you'll have it approved.