MGL Chapter 140, Section 121:
Firearms Definitions

Massachusetts General Laws Chapter 140, Section 121^[1] defines terms for Sections 122 through 131Y unless the context requires otherwise. It is not a definitional section for every subject covered anywhere in Chapter 140.

Core Definitions

Section 121 defines the foundational terms used throughout Chapter 140:

• Firearm: Under the post-Chapter 135 definition, "firearm" means a stun gun, pistol, revolver, rifle, shotgun, sawed-off shotgun, large capacity firearm, assault-style firearm, and machine gun, loaded or unloaded, which is designed to or may readily be converted to expel a shot or bullet, and includes the frame or receiver of any such firearm or the unfinished frame or receiver of any such firearm. Antique firearms and permanently inoperable firearms are excluded. The current definition has no barrel-length criterion; barrel lengths instead define "rifle" (16 inches or more) and "shotgun" (18 inches or more, with an overall length of 26 inches or more). This definition is substantially broader than the pre-Chapter 135 version, which required only that "a shot or bullet can be discharged" and did not cover frames, receivers, or unfinished components.
• Rifle: A weapon having a rifled bore with a barrel length of 16 inches or more.
• Shotgun: A weapon having a smooth bore with a barrel length of 18 inches or more and an overall length of 26 inches or more.
• Large capacity firearm: A secondary designation for a firearm that is semiautomatic with a fixed large capacity feeding device, is semiautomatic and capable of accepting or readily modifiable to accept a detachable large capacity feeding device under the statutory possession-or-control rule, employs a rotating cylinder holding more than 10 rounds or more than 5 shotgun shells, or is an assault-style firearm. Manual-action, single-shot, permanently inoperable, antique, relic, and theatrical-prop exclusions apply.
• Large capacity feeding device: A fixed or detachable magazine, belt, drum, feed strip, similar device, or complete set of parts that holds or can readily be converted to accept more than 10 rounds or more than 5 shotgun shells. Permanently altered devices below those limits, attached tubular .22-rimfire devices, lever-action tubular magazines, and pump-shotgun tubular magazines are excluded.

Chapter 135 Additions

Chapter 135 of the Acts of 2024^[2] introduced several new definitions that substantially expanded the scope of Section 121:

• Assault-style firearm: Defined through functional feature tests, a roster maintained under Section 131 3/4, a retained named-model list, and a copy-or-duplicate clause. The copy clause has a July 20, 2016 exclusion for qualifying firearms sold, owned, and registered before that date. Clause (g) contains the statute's manual-action, inoperable, antique, relic, theatrical-prop, federal Appendix A, and limited-shotgun exclusions.
• Untraceable firearm (ghost gun): A firearm that has not been serialized or whose identifying number has been removed, defaced, altered, obliterated, or mutilated. A valid serial number may come from a federal licensee, ATF, or DCJIS, so a properly serialized privately made firearm is not untraceable.
• Covert firearm: A firearm in a camouflaging container, or a firearm other than a stun gun that can fire and is shaped so it does not resemble or is not immediately recognizable as a firearm. The definition lists examples such as zip guns and firearms disguised as common objects.
• 3D printers and CNC milling machines: Section 121 does not define these devices. Section 121D regulates their use to manufacture or assemble firearms and restricts machines marketed or promoted primarily for that purpose.
• Unfinished frame or receiver: A forging, casting, printing, extrusion, machined body, or similar item that may readily be completed or assembled to function as a frame or receiver, or that is marketed or sold to become one when completed. Components for antique firearms are excluded.

Practical Significance

Because every prohibition, licensing requirement, and penalty in Chapter 140 references Section 121, changes to the definitions directly alter the scope of the law. The Chapter 135 expansion of the definition section means that provisions like the assault-style firearm prohibition (Section 131M), untraceable firearm regulations (Sections 121C and 121D), and storage requirements (Section 131L) all draw on the new terminology. The shift from "can be discharged" to "designed to or may readily be converted to expel" eliminates the prior operability requirement that Massachusetts courts enforced for decades, under which a broken or inoperable weapon was not a "firearm." Under the new standard, a weapon that was designed to fire, or one that can be readily restored to firing condition, is a firearm regardless of its current operational state. Neither "readily converted" nor "permanently inoperable" is defined in the statute.