MGL Chapter 140, Section 121:
Firearms Definitions

Massachusetts General Laws Chapter 140, Section 121^[1] is the definitional section for the entire firearms chapter. Every licensing, possession, and penalty provision in Chapter 140 relies on the terms defined here.

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 semiautomatic handgun or rifle that is capable of accepting more than 10 rounds of ammunition, or a shotgun capable of accepting more than 5 shells. Chapter 135 renamed this term from "large capacity weapon" to "large capacity firearm" and expanded the definition to include the frame or receiver of any such firearm.
• Large capacity feeding device: A magazine, belt, drum, feed strip, or similar device capable of accepting more than 10 rounds of ammunition.

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: Replaces the former "assault weapon" term. Defined by both functional characteristics (clauses a through c) and a retained named model list (clause e), broadening the scope beyond the prior definition.
• Untraceable firearm (ghost gun): Any firearm, frame, or receiver that lacks a serial number assigned by a licensed manufacturer or importer, or has had its serial number altered, removed, or obliterated. The statute uses "untraceable firearm" and "privately made firearm" as the defined terms; "ghost gun" is the common name.
• Covert firearm: A firearm designed or modified so as not to be readily recognizable as a firearm, such as a weapon disguised as an everyday object.
• 3D printer: A device capable of producing a three-dimensional object from a digital model, included for the purpose of regulating the manufacture of firearm components.
• Unfinished frame or receiver: A forging, casting, printing, extrusion, or machined body that is designed and intended to be completed to function as the frame or receiver of a firearm.

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.