This is a controversial topic, even though it shouldn’t be. To begin, let’s look at how the AR-15’s parts are named. The most basic distinctions are noted as the upper and lower–getting into stripped lowers and 80% lowers can add some confusion to the controversy. But we’re here to help clear things up.
The upper of an AR typically contains the actual upper receiver, bolt carrier group, barrel, and some furniture and optics and/or sights. There’s nothing associated with an AR-15’s upper (or the part called the upper receiver) that can’t be bought and sold direct. These parts are not serialized.
The lower is comprised of the actual lower part of the rifle (or AR pistol), the trigger group, the magazine well, the buffer tube, stock, and grip. The (typically) aluminum part that connects most of these parts and holds them in place is known as the lower receiver.
The upper is not a serialized part and, at least by the legal definitions of the ATF, it is not a “firearm.” These parts are all available on complete rifles, individually at gun stores, and even online.
What is a stripped lower?
The lower, though, is different. While you can buy parts and furniture, when it comes to the small aluminum (or occasionally polymer) lower receiver, you have to follow the ATF’s rules about gun purchases—even if that small piece of aluminum is the only piece of the gun you are buying.
See the image above? That shell is–by ATF definitions–a firearm. Without it, the rest of the rifle is little more than a paperweight.
When the lower is just the piece of aluminum with no pins, parts, etc., it is referred to as a stripped lower receiver. A stripped lower is just that, stripped of its parts.
Many of us begin building AR-15s by buying a stripped lower. This is the one part that requires a form 4473 transfer from a FFL. Even when it is stripped of all of its parts, and not connected to the rest of the rifle, this is a firearm and it has to be treated as such.
But look at this image below–this is not a lower receiver (though it could be with some basic DIY).
The 80% lower
And here we meet our second designation. Complete lowers (or lowers that are almost ready to install on a working gun) all have to have a serial number that is recorded on any sale or transfer. But if you are making the gun yourself, there are different rules.
Gunsmithing is an American tradition, and you can do it yourself, at home, with no oversight from the government, so long as you are building the guns for your personal use. This means you don’t have to add serial numbers or fill out background checks.
The laws governing ownership by felons still apply. But if you can legally own guns, you can legally make them, too.
This is where the concept of the 80% lower comes in. Above, where we noted that a receiver that is complete, or almost complete, there should have been a footnote. This part is going to get complicated, so buckle up.
What is an 80% lower?
You could begin with a block of aluminum and a milling machine and end up with a lower receiver. There are a few companies that do that. But most don’t.
Most gun companies use forgings—big stamped blocks that look somewhat like a lower. They then begin cutting away until the lower emerges. Some companies don’t want to do that finish work, either, and buy their lowers with all of the milling and drilling complete. This process requires that the lowers get stamped with serial numbers and logged with the ATF.
But there’s a middle ground. Forgings can be milled, and even drilled, so long as they aren’t finished too much. In these instances, we judge how much of the finished work is complete by percentages.
A massive block of aluminum might be simply 1% complete. A fully finished, ready-to-install lower receiver with all the finish work complete and drilling and milling completed might be 100% complete. It is a sliding scale.
Near that 100% lower mark is 80% lower. A low B on most American grading scales. Missing 20% of what it needs to work. Close to being finished, but not complete.
The ATF has designated these as raw materials and not as finished parts, so they don’t have to be transferred with a Form 4473 and they don’t have to be serialized if you are finishing it yourself, for your own use. If you are going to buy one and do the next 20% yourself, the ATF considers your efforts to be those of a gunsmith.
Finishing an 80% lower
And finishing a lower does require a modest amount of skill with some sort of device that removes metal (or sometimes polymer). Many home-smiths use a Dremel tool and a drill. These are common enough in most home workshops.
Those with a few more tools may employ a bigger rotary tool, or a drill press, or even a milling machine. No matter how you approach it, there will be some material that needs to be removed from the receiver and some holes drilled for pins. After that, the receiver should be ready for finishing.
The process isn’t idiot-proof, but it is hardly complicated. Many companies offer jigs and detailed instructions that make the process much less complicated. An 80% lower jig is worth the extra cost. A jig will make the drilling precise and it doesn’t change the legal definition of the gun.
If you don’t buy an 80% lower with a jig, you’ll end up going it alone. That’s not impossible, especially if you have the ability to run a CNC machine. If not, go with the jig.
Building an AR-15
You can take apart a rifle and pull out its stripped lower and begin a new build. You can buy a stripped lower that has never been part of a working gun and begin with just that. Or you can pick up an 80% lower, spend a couple of hours in the shop or garage, and be ready to go.
Your reasons are your own. For most of us, though, building an AR-15 is easy to do. It is a graceful and rewarding way to get deeper into the process of how these guns work. And you’ll learn a tremendous amount about how to fix any problems that may arise.
If you are new to the process, try disassembling a fully assembled AR-15. Start with the basic field strip, then move on to the bolt carrier group, then the trigger group, and then–finally–take apart the barrel. If you keep a few YouTube videos open and accessible, you should be able to disassemble and reassemble with a modest set of tools.
The next level is building your own, from scratch. Start with a parts list and begin putting together the rifle or pistol build you would design if you had everything your way.
The 80% lower takes that one step further. Not only do you get the benefits of building an AR, but you’ll end up paying very close attention to the guns negative spaces and to where pins go. It is fun, too, so why not?
One Last Tip
If there’s anyone that knows the AR-15 platform, it’s the US military. As a special offer for our readers, you can get the Official US Army Manual for AR-15/M4/M16 right now – for free. Click here to snag a copy.