Building a Firearm from a Lower Receiver in Massachusetts

Can I legally build a firearm from parts in Massachusetts after Chapter 135?

Expanded Definitions

Chapter 135^[1] fundamentally altered the legal landscape for home-built firearms by expanding the definition of "firearm" in MGL c. 140, Section 121^[2] to explicitly include "the frame or receiver of any such firearm or the unfinished frame or receiver of any such firearm." An "unfinished frame or receiver" is defined as a forging, casting, printing, extrusion, or machined body that either has reached a stage when it may readily be completed to function as a frame or receiver, or is marketed or sold to become one. This mirrors and in some respects exceeds the federal ATF Frame/Receiver Rule (27 CFR Section 478.12, effective August 24, 2022), which the Supreme Court upheld 7-2 in Bondi v. VanDerStok, 604 U.S. 458 (2025).

AR-15 Builds Are Effectively Prohibited

Since the AR-15 lower receiver is "the same as or interchangeable with the receiver of an enumerated firearm" (the Colt AR-15 on the Section 131M banned list), any firearm built on an AR-15 lower will almost certainly be classified as a "copy or duplicate" of an enumerated assault weapon. GOAL explicitly warns: "Private builds for any firearm that would fall under the new semi-automatic Assault Style Firearm definitions are prohibited after August 1, 2024." The interchangeability test, codified from AG Healey's 2016 Enforcement Notice, makes this determination nearly automatic for the AR platform.

The Grandfathering Question

Which grandfathering date applies to a home-built firearm remains genuinely unclear. A stripped lower purchased before August 1, 2024, but never assembled was not a "semi-automatic firearm" at that point. Yet under the expanded definition, it was a "firearm" (as a receiver). GOAL states there is "currently no clarity on how frames or receivers are being treated." EOPSS Guidance #4, Q27 addresses only the procedural question: "If I have registered a frame or receiver, do I need to re-register the firearm once it's been built? Yes." No guidance addresses the substantive legality question.

Serialization Requirements

New serialization requirements under Section 121C^[3] mandate that all firearms have serial numbers "conspicuously engraved, cast, or permanently embedded" with minimum depth of 0.003 inches and minimum print size of 1/16 inch, "placed in a manner not susceptible to being readily obliterated." GOAL argues this standard is so strict that "virtually no existing firearm meets it." Non-metallic frames (3D-printed) must use a permanently embedded metal plate.

3D Printers and CNC Machines

Section 121D^[4] regulates 3D printers and CNC machines. No person may use one to manufacture a firearm without a valid LTC. The sale or transfer of 3D printers or CNC machines with the "primary or intended function of manufacturing or assembling firearms" is prohibited. The test is whether the device is "advertised, marketed or promoted to manufacture or assemble firearms." Exemptions exist for forensic labs, law enforcement, and federally licensed manufacturers. Penalties run up to 1 year imprisonment and $5,000 fine per firearm. There is no registration requirement for general-purpose 3D printers. The law regulates use and sale of purpose-marketed devices.

Ghost Guns

"Ghost guns" (unserialized, untraceable firearms) are per se illegal under the "untraceable firearm" definition in Section 121. GOAL warns that "possession of an unserialized unfinished frame or receiver is punishable by up to 2.5 years in prison even if licensed." Several pre-Chapter 135 prosecutions signal enforcement priorities: Commonwealth v. Michael Roby (Norfolk Superior, approximately 2021) resulted in a 2.5-year sentence for possessing six ghost guns. Commonwealth v. Victor Pagliuca (Suffolk Superior, approximately 2020) involved two ghost guns and 3D mold materials.

The pending Hanlon v. Campbell (D. Mass., filed August 2025), brought by GOAL and the NRA, challenges the ASF provisions and is directly relevant to frames and receivers. GOAL has indicated the law must be challenged "in chunks by specific subject matter" and additional cases are planned.

Bottom Line

Settled: Unfinished frames and receivers are now legally "firearms" under Massachusetts law. AR-15 platform builds are effectively prohibited after August 1, 2024. Ghost guns are per se illegal. 3D printers and CNC machines marketed for firearm manufacture are regulated.

Unsettled: How grandfathering dates apply to stripped lowers purchased before August 1, 2024. Whether the serialization standard in Section 121C renders all commercially manufactured firearms technically non-compliant.

Do: Do not begin a new AR-platform build. If you possess a pre-August 1, 2024 stripped lower, consult a firearms attorney before proceeding.