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, and licensing 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 to function as one or is marketed or sold to become one. 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. Official Guidance Letter #4 contains inconsistent transition dates for older unserialized firearms, so a person should not wait to verify the current DCJIS procedure.

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.^[4] It separately restricts sale or transfer of machines marketed or promoted primarily for firearm manufacture. Section 121D does not create a registry for general-purpose equipment.

Current Federal Rule

Current 27 C.F.R. Section 478.12 includes certain readily completable frames, receivers, and weapon-parts kits within the federal definition, 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.
• Obtain the required serial number before manufacturing or assembling a privately made firearm.
• Use a qualified Massachusetts firearms attorney for an uncertain platform or grandfathering question.