Building an AR-15 (or any firearm) is an exciting project that new and experienced shooters can take on. If you're looking into building a black rifle or pistol from scratch, you've probably heard of the 80% lower receiver. This unfinished firearm part is the key to fabricating a custom AR or other gun. But what is it? Here's your guide.
DISCLAIMER: We are not attorneys. State and federal firearm laws change frequently, and the products discussed herein are banned in some states. This guide cannot ever be taken as legal counsel. If you are unsure about the legality of buying, building, or owning any firearm or any gun parts in your state, we encourage you to perform your own research about relevant state and federal laws.
What's an 80% lower?
An 80% lower is an un-serialized receiver blank, an unfinished firearm that is not operable and cannot be made to operate without additional fabrication. An un-finished 80% lower cannot accept a trigger, hammer, or safety, so it can't be made to fire a live round. Because of this, 80% lowers and all other types of receiver and firearm blanks aren't legally considered firearms by the Bureau of Alcohol, Tobacco, and Firearms (BATFE). State laws vary, which we'll touch on below.
The average gun owner can fabricate an 80% lower into a functional firearm using common tools and with little technical knowledge. The most common type of 80% lower receiver is made for the AR-15, like the forged and anodized lower above. It is an unfinished version of the AR's stripped lower receiver. These lowers (and 80% frames) exist for other rifles and weapons, like the AR9, AR-10, 1911, Sig Sauer P320, and Glock® series of handguns.
What's a stripped lower receiver?
A stripped lower receiver is the firearm component of the AR-15, the part of the weapon that is considered a gun under federal law. It has the necessary holes drilled and material removed by the manufacturer so parts can be installed. In its current state, it can function as a firearm. The 80% lower is the builder's alternative to buying a stripped receiver from a gun store. Cutting and drilling a receiver blank for an AR15 turns it into a stripped lower.
80% vs. stripped receivers
Now you know how stripped receivers are different from 80 lowers: One's a firearm, and one is not. But what are the other differences between the two?
These are the things you need to do to an 80% lower to make it work:
- Drill the hammer pin hole
- Drill the trigger pin hole
- Drill the safety selector lever hole
- Machine the fire control group cavity
These are the parts of the receiver blank the manufacturer already completed:
- Bolt catch
- Pistol grip hole
- Magazine well
- Magazine release
- Buffer detent hole
- Trigger guard pin holes
- Upper receiver rear lug pocket
- Front and rear takedown holes
- Buffer tube threads and housing
Why build an 80% lower?
There are advantages that come with building an 80% lower instead of buying a stripped receiver for a custom build. Note these advantages only apply to federal laws. Some states have enacted laws restricting 80% lowers.
- An 80% lower doesn't require a serial number. Federal law says that a firearm made for personal use does not require a serial number, though it is recommended (source). Again, state laws vary and some states require serialization for all receiver blanks before fabrication.
- An FFL isn't required to purchase one. Unlike firearms, buying a receiver blank doesn't require you to go through a Federal Firearm Licensee (FFL) or gun dealer to process the transaction. Since they're not firearms, 80% lowers can be bought and shipped like regular products under federal law.
- A background check isn't required. Federal law doesn't require a background check to purchase a receiver blank. Since you don't need to purchase one through an FFL, you don't have to fill out a Form 4473 (Firearm Transaction Record) like you would for a regular gun sale.
- They ship direct. No middleman like a dealer means your receiver blank can be shipped directly to your home or address of record. They are, however, not legal for international sale or shipment outside the United States.
- They're more affordable. Although you need some tools to get the job done, buying a receiver blank is often less expensive than buying that firearm's retail counterpart. That's because there are no gun dealer overhead costs or extra fees and taxes associated with the transaction.
How the law defines 80% lowers
Using federal law and agency policy, the ATF determines whether an 80% lower meets the definition of a firearm. They issue a written judgment called a Determination Letter for each make and model. The ATF has clarified that 80% lowers and unfinished firearm receivers are legal to buy, own, and fabricate under federal law without a license and for personal use only, if they do not meet the definition of a firearm. These firearm definitions are contained in the Gun Control Act (GCA).
The agency has provided this example of an unfinished 80% lower for the AR-15. It's being compared to a partially-machined lower, which they legally consider a firearm. Partially fabricating any type of receiver blank, to include drilling a single pin hole, may constitute making a firearm. You should not begin to machine an 80% lower unless you are certain that you are operating within the confines of all state and federal laws.
Some states have banned 80% lowers
Receiver blanks are not legal in all fifty states. Some states ( like California) require blanks to be serialized before fabrication. California residents must also submit to a background before building. Other states have outright banned the sale, ownership, fabrication, and use of receiver blanks. Because of changing firearm laws, this list may not be all-inclusive. These are the states that have banned receiver blanks:
- New Jersey
- New York
- Washington state
- Washington, D.C.
- Rhode Island
How does an 80% lower work?
An unfinished 80% lower cannot be used to make a working firearm. You must use tools to cut and drill excess material from the receiver blank, turning it into a functional stripped receiver or frame. Once fabrication has been completed, parts can be installed and the weapon can be made to fire. Fabricating a receiver blank can be accomplished with a tool kit called an 80% lower jig.
What's a jig?
The jig is a specialized piece of equipment that assists the builder with the fabrication process. It's made from steel, aluminum, or polymer and clamps around the receiver blank. Drill bits, end mill bits, and written or visual instructions are provided. With the bits and a drill press or mill, the jig will guide you along each fabrication step. You'll drill the holes and cut the cavity required for the receiver's internal parts.
What can I build with 80% lowers?
The most popular 80% lower is designed for the AR-15. The original AR has many variants, available configured as rifles or pistols. In the past few years, 80% frames have now been made for different handguns. This is a current list of available 80% lowers.
- LR-308 (AR-10)
- Sig Sauer P320
Q: Does an 80% lower need a serial number?
A: Under federal law, no. The ATF recommends a unique serial number be applied to your homemade firearm. That way, it can be traced more easily in the event it is lost or stolen. Some states require serialization, like California.
Q: Are 80% lowers different from their guns' retail receivers?
A: Once fabricated, an 80% lower is identical in form and function to its firearm platform's corresponding retail receiver or frame. This ensures parts can be installed and the weapon can be customized.
Q: Can I sell a completed 80% lower?
A: Yes, but with restrictions. You cannot ever make any receiver blank into a firearm with the intent of selling it for profit. This constitutes manufacturing, which is always illegal without a Federal Firearms License. Federal law does allow a homemade firearm to be sold later, on an individual transaction basis, without a Federal Firearms License.
Q: Can I complete an 80% lower without a jig?
A: You can, but only with advanced machining experience, complex calculations, and CAD software. Normally, building any firearm from raw materials would be prohibitively expensive. An 80% jig allows the average user to fabricate more easily.
Q: What tools do I need for this project, exactly?
Q: How hard is it to fabricate a receiver blank?
A: Basic knowledge of hand tools and power tools is typically required. You will need to use a drill press, mill, and other basic equipment to cut and drill a receiver blank. Polymer 80% lowers are great for first-timers, since they're easier to machine.
Q: What else do I need to assemble a firearm with a receiver blank?
A: Receiver blanks are sold separately from the other parts required to fabricate a gun. In addition to the lower, you will need a parts kit. An AR-15 build kit can be paired with a fabricated lower to make a functioning rifle.