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How do I finish an 80% lower?

You do not need to have any machining experience in order to finish our 80% lower receivers. All you need is a drill press, a few drills and end mills, and the 80% Arms Jig. You can view our step by step guide by clicking here.

Am I required to engrave a serial number onto my 80% lower?

Per federal law, an individual building a firearm for personal use is not required to mark it with a serial number.

Some states may have more restrictive regulations. The following is for California residents ONLY: In July, 2016, California passed AB 857 which requires all completed firearms to have a serial number applied by Jan 1, 2019. An 80% lower is not a firearm, so a serial number would only be required once the 80% lower is completed. Unfinished 80% lower receivers do not need a serial number.

If you engrave a serial number of your choosing into your completed 80% lower prior to Jul 1, 2018, you will not be affected by this law and you do not need to notify any government agency that you built a firearm.  In other words, you can make up any serial number you want (as long as it meets ATF's definition of a serial number), engrave it on your receiver, and this will satisfy California’s legal requirement. You do not need to apply to the state, or tell them what your serial number is. Your lower can still legally remain completely off the radar. The important thing is you MUST engrave your serial number before July 1, 2018 to take advantage of this benefit.

After July, 2018, the law changes for the worse. If you build an 80% lower after July 1, 2018, you must FIRST apply to the California DOJ for a serial number, pay a fee, and they will then assign a serial number that you must apply to your firearm. In this case, you can’t choose your own serial number and your serial number will be in the state government’s data base. If you want to avoid falling under this law, make sure you engrave your own serial number to your completed 80% lowers before July 2018.

If you build an 80% lower and use your own “unregistered" serial number after July 2018, it would be a misdemeanor, although the state would have the burden of trying to prove in court that you did in fact build the lower after the July 2018 cut off date.

80% Arms offers an 80% lower receiver serialization service. We have an FFL license and can engrave a serial number on your completed 80% lower receiver that meets ATF requirements.

Do you have an ATF determination letter?

Yes, we have received a letter from the ATF certifying that they do not consider our 80% lower a firearm. You can download a copy of this letter here.

Is this legal?

Yes, in our home state of California, as well as almost every other state in the U.S., it is legal to build your own firearm for personal use, as long as you can legally own a firearm. You must build the lower yourself; it is not legal for somebody else to do it for you.

From the ATF:Individuals manufacturing sporting-type firearms for their own use need not hold Federal Firearms Licenses (FFLs). However, we suggest that the manufacturer at least identify the firearm with a serial number as a safeguard in the event that the firearm is lost or stolen. Also, the firearm should be identified as required in 27 CFR 478.92 if it is sold or otherwise lawfully transferred in the future. 


If I live in California, are completed 80% lowers subject to the CA Assault Weapons ban?
Do I need a bullet button?

Completed 80% lowers are subject to all the same laws that apply to any normal firearm. If you live in CA, you need to be careful to follow the laws related to the assault weapons ban. In general, if you build AR-15 style rifle with a pistol grip, you will need a "bullet button". There are also "featureless" configurations that do not require a bullet button. For more info on following CA laws, has a very helpful flowcart: 

What are ATF Requirements for Serializing a lower?

First, home built rifles for personal use are not required by federal law to have serial number. However, some people wish to engrave one anyway, or you may be required to engrave one if you live in California (see above).

For firearms manufactured in the United States by a commercial firearms manufacturer, federal law requires the following information to be present:

  1. A serial number (must not duplicate another serial number you have used before)
  2. Caliber. Most people write “Caliber: Multi”.
  3. Manufacturer's name. Generally this would be your first and last name.
  4. Manufacturer’s city and state. This would be the city and state in which you live.
  5. (Optional) If you have decided to assign a model number to your 80% lower, you must engrave it onto the lower. If you have not elected to make up a model number, you do not have to engrave one.

The above information must be engraved to a depth of .003 inches and the text size must be at least 0.125 inches high. 

80% Arms can assist you with engraving a serial number onto your complete or unfinished 80% lower that meets these requirements. We use a high powered laser engraver that will cleanly and precisely burn into any metal material.

Disclaimer: We are not lawyers. The above information is not intended to be legal advice. Please do your own research, consult a lawyer, and do not rely on the above information. Check your local laws and make sure to follow all of them.