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Do 80 Lowers Need Serial Numbers?

We’ll cut right to the chase here: Sometimes, yet. Mostly, no! Depending on what state you live in, you do not need to serialize your 80 lower once it’s complete. Under Federal Law – that’s the laws set by the ATF – any individual building a firearm at home for personal use does not need to mark their firearm with a serial number.

That’s the easy answer, but it’s not the whole answer. Keep reading.

Some states require COMPLETED 80% lowers to have serial numbers

Just like the title says, you might have to serialize your 80% lower before you actually turn it into a by-definition firearm. California, for example, is one state that requires this. They passed state legislation (AB 857) which requires all completed firearms to have a serial number applied by January 1, 2019. Technically, an 80% lower isn’t a firearm, but owning a firearm without a serial number will eventually be illegal in the state of California.


That means you need to serialize your 80% lower before you ever put an end mill bit to metal. Doing so requires contacting the California Department of Justice for a serial number. You must also pay a fee and wait for the CA DOJ to assign you the serial number you’ll need to engrave on your new rifle or pistol.


Unless machined, an 80% lower does NOT require a serial number

To answer the question of this article in a literal sense: No, an 80% lower does not ever need a serial number. Why? Because it’s not legally a firearm! An 80% lower, unless machined, cannot accept a hammer, trigger, or fire control group and thus, under the ATF’s own definition of what a firearm is, it is not considered such.


Legally, an 80% lower is no different than a large, cubic hunk of aluminum alloy. Unless the day comes that you need a serial number on the tin foil in your kitchen, you will not ever need a serial number on your 80% lower. Keep in mind the rules do change – obviously, in the case of California – once you’ve machined that 80 lower, so do your research before you turn on that router and spin that end mill bit.


What are the ATF’s requirements for serializing a firearm?

Excellent question! If you do, in fact, have to serialize your 80 lower, then the engraving requirements are pretty straightforward. You’ll need the serialization to measure .003” deep with text size at least 0.125” high. You’ll need the following information engraved on your lower, as well:


  1.       Serial number (it cannot duplicate any other serial number you may own or use)
  2.       Caliber (it’s best to choose “Caliber: Multi”)
  3.       Manufacturer’s name (that’s you!)
  4.       Manufacturer’s city and state (where do you live?)
  5.       OPTIONAL: Model number

We recommend using “multi” under “Caliber” if you decide to switch upper receivers and go from 5.56 to .300 BLK, or some other conversion. Electing a model number is optional unless you live in California and intend to register your new rifle or pistol as an assault weapon.

DISCLAIMER: If you are new to the world of DIY AR-15 building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. We are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.

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