I just moved out of MA in April 2009. I escaped across the border into NH.
So yes, I know the MA laws quite well. I had a Class A license and even a machine gun license in MA.
Preban rifles do not need to have been in MA before the ban. Some MA FFLs will not transfer them b/c they do not want any potential harrassment from the MA AG, but it is perfectly legal. Same thing with preban mags. So if you want to get preban rifle and mags in NH and bring them home to MA, that is fine.