What Exactly is an 80% Lower?
An 80% lower is a piece of aluminum (or polymer) that has been machined to a point where it is not yet legally considered a “firearm.” It lacks the trigger pocket and the holes for the safety and trigger pins. Because it is technically just a “hunk of metal,” it can be shipped directly to your house without a background check or an FFL transfer in many states.
You Are the Manufacturer
When you finish an 80% lower, you are legally the “manufacturer” of that firearm. Under federal law (in the US), you are allowed to make a firearm for personal use. However, you cannot manufacture it with the “intent” to sell it. If you ever decide to sell it later, you must follow specific federal and state laws regarding serialization and transfer. Always check your local regulations before starting.
The Necessity of a Jig System
You cannot finish an 80% lower by “eyeballing” it with a hand drill. You need a “Jig.” A jig is a steel or aluminum template that bolts onto the lower and guides your drill bits and router. High-quality “Router Jigs” are the gold standard today, as they allow you to create a factory-smooth trigger pocket that looks like it was done on a professional CNC machine.
Co-Witness Iron Sight Heights
If you are running a red dot, you will likely need “Suppressor Height” iron sights. These are taller sights that allow you to see the irons through the window of your optic. When choosing 80% lowers slide, check where the rear sight “dovetail” is located. Some slides put the iron sight in front of the optic (to protect the glass), while others keep it in the traditional rear position.
Material Choices: Aluminum vs. Polymer
Aluminum 80% lowers (usually 7075-T6) are the most durable and offer the “classic” AR-15 feel. They require more time and better tools to finish. Polymer 80% lowers are much easier to mill and can often be done with just a hand drill and some snips. However, polymer is less durable in the long run, especially around the buffer tube threads where the most stress occurs during firing.
Legal Variations by State
While 80% lowers are legal under federal law, several states (like California, New York, and Washington) have passed their own restrictive laws. Some states require you to apply for a serial number from the state government before you start milling. Others have banned the “kits” entirely. It is your responsibility to stay up to date on your specific state’s “Ghost Gun” legislation to avoid serious legal trouble.
The Required Tooling Investment
Building from an 80% lower isn’t necessarily a way to “save money.” By the time you buy the lower, the jig, the specialized drill bits, and a router, you could have bought several finished lowers. The real value is in the privacy, the “educational experience” of building your own tool, and the pride of craftsmanship. Don’t go into this project thinking it’s a budget shortcut.
Importance of the “Trigger Pocket” Depth
The most common mistake when milling an 80% lower is not going deep enough in the trigger pocket. If the pocket is too shallow, the safety selector won’t fit, or the hammer won’t have enough room to swing. Follow your jig’s instructions to the millimeter. Using a “depth gauge” or a set of calipers is essential to ensure the internal dimensions match the Mil-Spec requirements for a functioning firearm.
Safety and Responsibility
Once you finish that lower, it is a “real” firearm. It has no “built-in” safety other than the one you install. You must treat it with the same (or more) respect as any store-bought gun. Because it lacks a serial number in many jurisdictions, it is even more important to keep it locked in a secure safe. Building your own firearm is a high-level exercise in the Second Amendment, and it carries heavy legal and moral responsibility.