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AB 857 and Building an 80% Lower for Your 300 Blackout in CA

Posted by 80-lower on Nov 3rd 2019

AB 857 and Building an 80% Lower for Your 300 Blackout in CA

California's gun laws are an infuriatingly confusing and complex minefield, aren't they? Let's walk you through navigating that mine field, so you can legally build your own 80% lower and 300 Blackout rifle. Now, this article won't dive into the nuances of what makes an AR 15 (regardless of caliber) legal in California - that's an entirely different topic all itself.

What we will detail are California's latest gun laws that relate to finishing an 80% lower - the biggest roadblock to homebuilding your own AR 15 in 300 Blackout from start to finish. There is one major law that affect this hobby: AB 857.

First, a disclaimer: We're not law makers or lawyers, nor can we possibly provide actual legal advice to you, oh hopeful AR builder. Take this information and use it as a foundation, but seek actual legal counsel if you're uncertain about anything.

AB 857 and What it Means For Black Rifle Builders

California's so-called AB 857 legislation means that all finished firearms, made in a factory or your garge, must have a serial number by January 1, 2019.

It also means you have until July 1, 2018 to buy 80% lowers and finish them without any paperwork. On that date, you'll have to report the finished lower to the California Department of Justice.

Lastly, AB 857 means if you finish an 80% lower into a legally defined firearm after July 1, 2018, you'll need to apply for a serial number from the California DOJ. You'll also need to pay a fee for said service, and engrave said serial number on your lower within 10 days, or you'll face a misdemeanor charge. Whew.

Why AB 857 Doesn't Matter That Much

Here lies the rub: If you finish an 80% lower before July 1, 2018, you'll still have to engrave it with a serial number. But, you won't have to report the receiver or the serial number to the California Department of Justice.

Now, listen carefully. In our opinion (and this is the opinion of many), there is no reasonable way for the California DOJ to prove that you built your 80% lower after the deadline of July 1, 2018. Take that with a grain of salt and let the implication sink in. Now, if you were to have built said 80 lower receiver before the deadline, you wouldn't have to apply for a serial number, you wouldn't have to pay for anything, and you wouldn't have to report said finished lower to the CA DOJ.

Premium Billet 80% Lower Fire/Safe Marked (1-Pack) & Easy Jig Gen 2 with Tooling

Just make sure you have a serial number on that bad boy before you even finish it and slap a parts kit in there for your 300 Blackout build. That's all we'll say.

Serializing Your 300 Blackout Lower

The serial and markings on your 300 Blackout lower have to follow some strict guidelines in order to be a legal firearm in California, or any state:

  • Markings must be at least .003" deep
  • Marking font size cannot be smaller than 1/16"
  • Markings must use Roman letters and Arabic numbers only
  • Markings must include a serial number
  • Markings must include the weapon model
  • Markings must include the caliber
  • Markings must include the maker;s name (that's you!)
  • Markings must include the city and state of manufacture

Serializing your lower can be a messy process if you try doing it at home without the right tools. There are aluminum engraving tools available, but these are expensive and usually not worth the one-time cost if you're an at-home builder. We recommend taking your unfinished lower to a CNC shop or metallurgist so they can serialize it before you begin machining yourself.

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.