Building a Firearm from a Lower Receiver in Massachusetts

Can I legally build a firearm from a lower receiver in Massachusetts? The answer depends on whether the item is already a firearm or unfinished frame or receiver under Massachusetts law, whether the planned firearm would be prohibited, and whether serialization, registration, licensing, and relevant state and federal safety requirements can be satisfied. A marketing label such as "80 percent" does not decide the Massachusetts classification.

Massachusetts Definitions

Section 121 includes frames, receivers, and unfinished frames or receivers in the definition of firearm.^[2] An unfinished frame or receiver is covered when it may readily be completed or assembled to function as one or is marketed or sold to the public to become or be used as the frame or receiver of a functional firearm once completed or assembled. The definition excludes a component designed and intended for use in an antique firearm. The statutory test, rather than a claimed completion percentage, controls.

Serialization and Registration

Section 121C generally requires firearms to bear a valid serial number.^[3] A person manufacturing or assembling a privately made firearm must obtain a serial number before manufacture or assembly, place it on the firearm during that process, and register the completed firearm within seven days. EOPSS Guidance #5 states that individuals are not required to comply with the registration and serialization requirements of Sections 121B and 121C until October 28, 2026, and that firearms manufactured before October 22, 1968 are exempt from serialization.

Assault-Style Firearm Restrictions

Chapter 135 expanded the assault-style-firearm definition and Section 131M restrictions.^[1] Whether a particular lower and planned configuration is prohibited is a fact-specific classification question. The statute's named-platform, copy-or-duplicate, feature, grandfathering, and registration provisions must all be checked. The site does not treat every stripped lower as automatically lawful or automatically prohibited.

3D Printers and CNC Machines

Section 121D requires a valid Massachusetts LTC to use a 3D printer or CNC milling machine to manufacture or assemble a firearm, except for equipment exempted under subsection (c).^[4] Subject to the exceptions in Section 121D(c), it prohibits selling, offering to sell or transferring to any person in the Commonwealth a 3D printer or CNC milling machine advertised, marketed or promoted to manufacture or assemble firearms, even if it is also described as general-purpose equipment. Section 121D does not create a registry for general-purpose equipment.

Current Federal Rule

Current 27 C.F.R. Sections 478.11 and 478.12 include certain readily completable weapon parts kits and frames or receivers within their respective federal definitions, while excluding some raw or unindexed blanks depending on their condition and circumstances.^[5] ATF stated in May 2026 that the frame-or-receiver rule remained under legal review and that any change would be published as a proposal subject to notice and comment.^[6] A review is not a repeal of the current rule.

Bottom Line

• Do not rely on an "80 percent" label as a legal classification.
• Confirm the planned firearm is not prohibited before beginning work.
• When Section 121C applies, obtain a unique serial number from DCJIS before manufacturing or assembling a privately made firearm. EOPSS Guidance #5 states that individuals are not required to comply with Section 121C's serialization requirements until October 28, 2026.
• Use a qualified Massachusetts firearms attorney for an uncertain platform or grandfathering question.