As to why the Streetsweeper/Striker-12/USAS-12 are considered "non-sporting":
Anyone really interested in this stuff should go read about WHY this ruling was made, and read the actual ruling itself. It was at the request of Clinton-era (ruling was from 1994) Brady Bunch moonbats. They requested that the above guns, along with the Mossberg 500 Bullpup be classified as DDs. The Mossberg 500 was not declared a DD, and the others where, because of one simple reason.
Its not possible to LIMIT the capacity of them. Federal law doesn't define "sporting purposes", but it does establish rules for hunting on Federal land. That law requires that shotguns be limited to 3 rounds. There is no good way to limit the revolver-type guns to 3 rounds, and Gilbert/Daewoo never made or sold a small mag for the USAS. The Mossberg used a normal 500 mag tube, which is easily limited.
A good reason the Saiga 12 won't become a DD is because 2 round (+1 in the gun) mags are available.
As to the legality of USAS-12s in MA:
No problem! Only thing to keep in mind is that even your average Class 3 dealer can't transfer them. ATF requires that dealers of DDs have a Type 9, 10 or 11 FFL. Usually NFA branch will process the transfers from non-DD FFLs, as they will all be tax paid, but the next compliance inspection will get the FFL in hot water. Ask me how I know!
As far as DD vs MG: MG trumps all, including caliber. A Mk19 40mm grenade launcher is a MG. If anyone made one, and I'm sure there are a few, an approved by FTB, semi-auto only Mk19, it would be a DD.
--EasyD