Bottom line: carrying one on your person on a public way—or simply keeping one loose or accessible in your vehicle—can expose you to serious criminal charges.

M.G.L. – Section 121, Chapter 140

''Large capacity feeding device'', (i) a fixed or detachable magazine, belt, drum, feed strip or similar device that has a capacity of, or that can be readily converted to accept, more than 10 rounds of ammunition or more than 5 shotgun shells; or (ii) any part or combination of parts from which a device can be assembled if those parts are in the possession or control of the same person; provided, however, that ''large capacity feeding device'' shall not include: (a) any device that has been permanently altered so that it cannot accommodate more than 10 rounds of ammunition or more than 5 shotgun shells; (b) an attached tubular device designed to accept and capable of operating only with.22 caliber rimfire ammunition; or (c) a tubular magazine that is contained in a lever-action firearm or on a pump shotgun. 

M.G.L. – Section 131, Chapter 140

(a) A license shall entitle a holder thereof of a license to purchase, rent, lease, borrow, possess and carry firearms, including large capacity firearms and ammunition therefor. The license shall not entitle a holder thereof to transfer, possess or carry large capacity feeding devices or assault-style firearms unless such transfer, possession or carry is permitted under section 131M. 

M.G.L. – Section 131C, Chapter 140

Section 131C. (a) No person carrying a loaded firearm under a license issued pursuant to section 129B 131 or 131F or through an exemption under section 129C shall carry the loaded firearm in a vehicle unless the loaded firearm while carried in the vehicle is under the direct control of the person. Whoever violates this subsection shall be punished by a fine of $500.

(b) No person possessing a large capacity rifle or shotgun under a license issued pursuant to section 131 or 131F or through an exemption under section 129C shall possess the large capacity firearm in a vehicle unless the large capacity firearm is unloaded and secured in a locked container as defined in section 121. Whoever violates this subsection shall be punished by a fine of not less than $500 nor more than $5,000.

(c) This section shall not apply to: (i) an officer, agent or employee of the commonwealth, any state or the United States; (ii) a member of the military or other service of any state or of the United States; (iii) a duly authorized law enforcement officer, agent or employee of a municipality of the commonwealth; provided, however, that a person described in clauses (i) to (iii), inclusive, is authorized by a competent authority to carry or possess the firearm so carried or possessed and is acting within the scope of the person's official duties.

(d) A conviction of a violation of this section shall be reported immediately by the court or magistrate to the licensing authority. The licensing authority shall immediately revoke the firearm identification card or license of the person convicted of a violation of this section. No new firearm identification card or license may be issued to a person convicted of a violation of this section until 1 year after the date of revocation of the firearm identification card or license. 

M.G.L. – Section 131M, Chapter 140

Section 131M. (a) No person shall possess, own, offer for sale, sell or otherwise transfer in the commonwealth or import into the commonwealth an assault-style firearm, or a large capacity feeding device.

  (b) Subsection (a) shall not apply to an assault-style firearm lawfully possessed within the commonwealth on August 1, 2024, by an owner in possession of a license to carry issued under section 131 or by a holder of a license to sell under section 122; provided, that the assault-style firearm shall be registered in accordance with section 121B and serialized in accordance with section 121C.

  (c) Subsection (a) shall not apply to large capacity feeding devices lawfully possessed on September 13, 1994 only if such possession is: (i) on private property owned or legally controlled by the person in possession of the large capacity feeding device; (ii) on private property that is not open to the public with the express permission of the property owner or the property owner's authorized agent; (iii) while on the premises of a licensed firearms dealer or gunsmith for the purpose of lawful repair; (iv) at a licensed firing range or sports shooting competition venue; or (v) while traveling to and from these locations; provided, that the large capacity feeding device is stored unloaded and secured in a locked container in accordance with sections 131C and 131L. A person authorized under this subsection to possess a large capacity feeding device may only transfer the device to an heir or devisee, a person residing outside the commonwealth, or a licensed dealer.

  (d) Whoever violates 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 1 year nor more than 10 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 5 years nor more than 15 years, or by both such fine and imprisonment.

  (e) This section shall not apply to transfer or possession by: (i) a qualified law enforcement officer or a qualified retired law enforcement officer, as defined in the Law Enforcement Officers Safety Act of 2004, 18 U.S.C. sections 926B and 926C, respectively, as amended; (ii) a law enforcement officer as defined in section 1 of chapter 6E who is currently certified pursuant to section 4 of said chapter 6E; provided, however, that to qualify for the exemption under this subsection, the certification of a law enforcement officer as defined in section 1 of chapter 6E shall not be suspended, limited, or restricted and the officer shall be permitted to perform police duties and functions involving firearms in accordance with chapter 6E and the regulations promulgated thereunder; (iii) a federal, state or local law enforcement agency; or (iv) a federally licensed manufacturer solely for sale or transfer in another state or for export.