While I agree with your opinion it is interesting that C140 S121 says:
“Firearm”, a pistol, revolver or other weapon of any description, loaded or unloaded, from which a shot or bullet can be discharged and of which the length of the barrel or barrels is less than 16 inches or 18 inches in the case of a shotgun as originally manufactured; provided, however, that the term firearm shall not include any weapon that is: (i) constructed in a shape that does not resemble a handgun, short-barreled rifle or short-barreled shotgun including, but not limited to, covert weapons that resemble key-chains, pens, cigarette-lighters or cigarette-packages; or (ii) not detectable as a weapon or potential weapon by x-ray machines commonly used at airports or walk- through metal detectors.
So according to that MGL a SBR is NOT a firearm and we already know it's not a rifle (barrel is less than 16") and it's not a pistol (has stock). Also the feds actually defined what a SBR was but the commonwealth didn't when they slammed the AWB through before the federal law sunset. Again crappy law.