For gun owners interested in building their own firearms, 80% lowers offer an exciting way to create custom AR-15s, AR-10s, or other firearms while gaining a deeper understanding of their mechanics. But when it comes to serial numbers, the rules can get a bit confusing. Do you need to serialize an 80 lower? What does federal law require? And how do state regulations come into play?
In this article, we’ll break down everything you need to know about 80% lowers and serial numbers, so you can confidently navigate the legal requirements and stay compliant.
What Is an 80% Lower?
An 80% lower receiver is a partially finished firearm component that requires additional machining to become functional. Because it is incomplete, it is not legally classified as a firearm under federal law.
This distinction allows you to purchase an 80% lower without:
- A federal background check.
- An FFL (Federal Firearms License) transfer.
- A serial number (in most states).
Do 80% Lowers Require Serial Numbers?
Federal Law
Under federal law, firearms manufactured for personal use do not require a serial number, as long as they are not intended for sale or distribution. This means that you can legally complete an 80% lower receiver and assemble a firearm without adding a serial number—if it is for your personal use only.
However, the firearm must still comply with all other federal regulations, including restrictions on fully automatic weapons or other prohibited designs.
State and Local Laws
While federal law does not mandate serial numbers for personal-use firearms, several states and local jurisdictions have stricter rules. Here are some examples:
- California
- Requires serialization of all home-built firearms.
- Builders must apply for a serial number through the California Department of Justice (DOJ) before completing the firearm.
- Connecticut
- Mandates serialization for all firearms, including those built from 80% lowers.
- New York and New Jersey
- Prohibit the possession of unserialized firearms, effectively banning the completion of 80% lowers without serialization.
- Hawaii
- Prohibits the sale, purchase, or possession of 80% lowers entirely.
Know Your State’s Laws
If you live in a state that requires serialization, you must engrave your completed firearm with a serial number that meets specific size and depth requirements. Always check your state and local laws before purchasing or completing an 80% lower.
How to Serialize a Firearm Built from an 80% Lower
If you’re required to serialize your firearm, follow these steps:
1. Apply for a Serial Number (If Required)
Some states require you to apply for a serial number through their DOJ or another regulatory agency. Once approved, you’ll receive a unique number for your firearm.
2. Engrave the Serial Number
The ATF has specific guidelines for serial number engraving:
- Minimum Depth: The engraving must be at least 0.003 inches deep.
- Minimum Size: Characters must be at least 1/16 inch in height.
- Placement: The serial number must be engraved on the receiver where it is clearly visible.
3. Include Additional Information (If Required)
In some cases, you may also need to engrave:
- Your name (as the builder).
- The city and state where the firearm was manufactured.
Professional engraving services or licensed gunsmiths can help ensure compliance with these requirements.
Why Should You Consider Serializing Your Firearm Even If It’s Not Required?
Even if your state doesn’t mandate serialization, there are good reasons to add a serial number to your completed firearm:
- Theft Recovery
A serial number can help law enforcement identify and return your firearm if it is ever lost or stolen. - Proof of Ownership
A serialized firearm can serve as proof of ownership in case of disputes or legal issues. - Future Legal Compliance
Laws regarding serialization may change. By adding a serial number now, you can ensure your firearm remains compliant with potential future regulations.
What Happens If You Build an Unserialized Firearm?
If you complete an 80% lower and assemble a firearm without adding a serial number, you are generally in compliance with federal law—as long as the firearm is for personal use. However, there are risks:
Federal Violations
- Selling or transferring an unserialized firearm is illegal under federal law.
State Violations
- In states requiring serialization, possessing an unserialized firearm can result in criminal charges.
Always research and comply with your local laws to avoid serious legal consequences.
Can You Sell a Firearm Built from an 80% Lower?
Yes, but there are strict conditions:
- The firearm must be properly serialized.
- You must sell or transfer the firearm through a licensed FFL dealer.
- The buyer must pass a background check.
Selling a homemade firearm without meeting these requirements is a federal crime, even if serialization wasn’t required for personal use.
Common Questions About 80% Lowers and Serial Numbers
Do I Need to Register My 80% Lower?
Federal law does not require registration of 80% lowers or firearms built from them. However, some states, like California, mandate registration of home-built firearms.
Can I Add a Serial Number After I Finish My Build?
Yes, you can add a serial number at any time. However, if your state requires serialization, it’s best to complete the engraving before finishing your build to avoid potential legal issues.
Do Polymer 80% Lowers Need to Be Serialized?
The material of the lower doesn’t change the legal requirements. Whether you’re using aluminum or polymer, serialization depends on your state’s laws.
Final Thoughts
Understanding the rules around 80% lowers and serial numbers is essential for any gun owner looking to build a firearm. While federal law provides some flexibility, state and local regulations can significantly impact what’s required.
By staying informed, following the rules, and considering the benefits of serialization, you can confidently build and own a firearm that meets all legal requirements.
Looking for high-quality 80% lowers? Check out Thunder Tactical for everything you need to get started, from top-tier lowers to engraving services and jig kits. Start your build the right way!