These are specifically allowed to be owned by persons with a License to carry, as defined in MGL C140 S131 Para A which reads, in part""Large capacity feeding device'', (i) a fixed or detachable magazine, box, drum, feed strip or similar device capable of accepting, or that can be readily converted to accept, more than ten rounds of ammunition or more than five shotgun shells; or (ii) a large capacity ammunition feeding device as defined in the federal Public Safety and Recreational Firearms Use Protection Act, 18 U.S.C. section 921(a)(31). The term ""large capacity feeding device'' shall not include an attached tubular device designed to accept, and capable of operating only with, .22 caliber ammunition.
You can see the full text here: http://www.state.ma.us/legis/laws/mgl/140-121.htm(a) A Class A license shall entitle a holder thereof to purchase, rent, lease, borrow, possess and carry: (i) firearms, including large capacity firearms, and feeding devices and ammunition therefor, for all lawful purposes, subject to such restrictions relative to the possession, use or carrying of firearms as the licensing authority deems proper; and (ii) rifles and shotguns, including large capacity weapons, and feeding devices and ammunition therefor, for all lawful purposes;