Chapter 140 - 131N is why I'd be worried. (In full below)
Specifically, "No person shall... possess any weapon, capable of discharging a bullet.. that does not resemble a handgun"
Heck, in reading 131N, if it doesn't look like a SBR, SBS or handgun, it would be illegal. I would ASSuME that someone else standard rifles, shotguns and Machineguns are exempt from 131N.
Mike
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http://www.mass.gov/legis/laws/mgl/140-131n.htm
SALE OF FIREARMS
Chapter 140: Section 131N. Covert weapons; sale, transfer or possession; punishment
Section 131N. No person shall sell, offer for sale, transfer or possess any weapon, capable of discharging a bullet or shot, 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. Whoever violates the provisions of this section shall be punished, for a first offense, by a fine of not less than $1,000 nor more than $10,000 or by imprisonment for not less than one year nor more than ten years, or by both such fine and imprisonment, and for a second offense, by a fine of not less than $5,000 nor more than $15,000 or by imprisonment for not less than five years nor more than 15 years, or by both such fine and imprisonment.