Does My 80 Lower Need a Serial Number? How to Apply One (Updated 2019)

Oct 9th 2018

Does My 80 Lower Need a Serial Number? How to Apply One (Updated 2019)

Deciding whether to build your AR-15 using an 80% lower? You've probably got some legal questions, especially when it comes to serial numbers. All guns you've bought before have had a serial number imprinted on them for the sake of the background check and FFL paperwork.

Does your 80 lower need a serial number, too?

In most cases, you do not need to serialize your 80% lower, even after building your rifle or pistol.

Under federal law, any individual building a firearm at home for personal use does not need to mark their firearm with a serial number. This is what the ATF recommends:

"Additionally, although markings are not required on firearms manufactured for personal use (excluding NFA firearms), owners are recommended to conspicuously place or engrave a serial number and/ or other marks of identification to aid in investigation or recovery by State or local law enforcement officials in the event of a theft or loss of the privately owned firearm." You can read the statement here, on page 8.

State laws can vary. Or, maybe you want to sell that homemade gun later. Keep reading to learn when you do need a serial number, and how to do it:

Some states require finished lowers to have serial numbers

If you live in a restrictive state, you might have to serialize your 80% lower before you turn it into a firearm.

California is one example. The state passed legislation (AB 857) which requires all completed firearms to have a serial number:

This bill [will], commencing July 1, 2018, and subject to exceptions, require a person who manufactures or assembles a firearm to first apply to the department for a unique serial number or other identifying mark, as provided. The bill [will], by January 1, 2019, and subject to exceptions, require any person who, as of July 1, 2018, owns a firearm that does not bear a serial number to likewise apply to the department for a unique serial number or other mark of identification.

This means you need to serialize your lower before you drill a hole or cut any metal.

An unfinished 80% lower doesn't require a serial number

Unfinished 80% lowers don't need a serial number, ever. Even in California. Remember, they're not considered guns. Legally, an 80% lower is just 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 need a serial number on the 80% lower you just bought. Keep in mind the rules do change once you've machined that lower and made a gun (like California), so do your research before you cut or drill anything.

California 80% Lower Serialization Guide

(Updated September, 2019)

These are the steps you must take to serialize your 80% lower if you live in California:

  • Fill out a Personal Firearms Eligibility Check Application (PFEC)
  • Complete the PFEC and mail it with a $20 check or money order
  • Create an account on the California CFARS
  • Use your new CFARS account to submit a Unique Serial Number Application
    • The application can be paid for and submitted in CFARS
  • Wait to receive your serial number via email and CFARs

The whole application process takes a few weeks. California DOJ has to first approve your PFEC application, then approve your serial number application.

After getting your unique serial number, you only have a few days to get it engraved with pictures uploaded to CA DOJ via CFARS as proof.

Serial Number Engraving Requirements:

Whether living in California or elsewhere, these are the requirements you should follow when engraving your serial number:

The engraving needs to measure 0.003" deep. The text size must be at least 0.125" high.

You'll need the following information engraved on your lower:

  1. Serial number
  2. Caliber (it's best to use "Multi")
  3. Manufacturer's name (that's you!)
  4. Manufacturer's city and state (where do you live?)
  5. Model number (optional)

Who can engrave my lower?

Thankfully, anyone with the right tools can do the job. Because an unfinished 80% lower isn't legally a gun, you don't need to visit a gunsmith or FFL to have your serial number engraved. As long as the lower isn't completed, you can ask a friend, machinist, hardware store, or engraving company to do this for you.

If your lower is machined in any way, it may be legally considered a firearm. If that's the case, you must visit a licensed gunsmith or FFL to have your serial number engraved.


An unfinished 80% lower doesn't need a serial number. Ever. Finished 80% lowers don't need a serial number under Federal law. The ATF still recommends it in case your weapon is ever lost or stolen, or you wish to sell it later. If you live in California, you need to apply for a unique serial number and have it engraved before you finish your lower.

More questions about this stuff? We want to help! We build 80% lowers and AR-15s just like you. Give us a call or email us and we'll get in touch with you ASAP, usually a day or less.

DISCLAIMER: If you are new to the world of DIY gun 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. At, 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.