Untraceable Firearms and Serialization Requirements:
Sections 121C and 121D

Chapter 135 added Sections 121C and 121D to address firearm serialization and specified uses or sales of 3D printers and CNC milling machines.^[1] The sections regulate different conduct and do not create a general registry for manufacturing equipment.

Which Items Are Firearms

Section 121 includes frames, receivers, and unfinished frames or receivers in the firearm definition.^[4] An unfinished item is covered when it may readily be completed to function as a frame or receiver or is marketed or sold to become one. A label such as "80 percent lower" is not the statutory test, and a raw piece of material is not automatically a firearm merely because it could be machined.

Section 121C: Serial Numbers

Section 121C generally requires firearms to bear a valid serial number and prohibits specified conduct involving firearms without one. Individuals are not required to comply with Sections 121B and 121C until October 28, 2026, as explained below.^[2] A person manufacturing or assembling a privately made firearm must obtain a serial number before starting, place it on the firearm during manufacture or assembly, and register the completed firearm within seven days.

EOPSS Guidance #5, issued September 18, 2026, says individuals must comply with the registration and serialization requirements in Sections 121B and 121C beginning October 28, 2026.^[5] Firearms already registered through the transaction portal do not have to be registered again when the new system goes live, and firearms that already have a valid serial number do not need a new one. This transition date must be read alongside the statutory deadlines and applicable exemptions.

Section 121D: Equipment Use and Sale

Section 121D requires a valid LTC to use a 3D printer or CNC milling machine to manufacture or assemble a firearm.^[3] It also prohibits selling, offering to sell, or transferring a 3-dimensional printer or CNC milling machine that has the primary or intended function of manufacturing or assembling firearms. A machine has that function if it is advertised, marketed, or promoted to manufacture or assemble firearms, whatever other functions it has. Section 121D(c) lists exemptions. It does not require a person to register a general-purpose printer or mill with the state.

Penalties and Scope

Section 121C supplies the serialization requirements, while Chapter 269, Section 11C contains criminal penalties for specified untraceable-firearm conduct. Section 121D has its own penalties and exemptions. The classification of the item and the exact conduct determine which rule applies.

Current Federal Rule

The Supreme Court held in Bondi v. VanDerStok that ATF had statutory authority to regulate at least some weapon-parts kits and unfinished frames or receivers.^[6] Current 27 C.F.R. Section 478.12 remains in effect and applies to certain readily completable items, while its text excludes some raw or unindexed blanks depending on their condition and circumstances.^[7]

Practical Compliance

• Use the statutory tests, not an "80 percent" marketing label.
• Obtain the required serial number before making a privately made firearm.
• Register the completed firearm within the statutory period.
• Do not assume federal compliance establishes Massachusetts compliance.