We know why you’re here. You’re ready to exercise a Second Amendment right that few people ever use.
You want to build your own gun at home.
Even better, you want to build a kick-ass AR-15 using something called an 80 percent lower.
This guide assumes you know the basics already. It’s going to teach you what to look out for when you buy your first receiver blank. This guide covers material, cost, type, and caliber.
First, get to know the laws
Some good news: Building your own gun at home is perfectly legal in almost every state, and you probably won’t need to fill out any paperwork. A few states (like California and Washington) do have some laws in place about this sort of stuff and yes, there will be paperwork to file in those states.
Depending on where you live, you’ll need to get your receiver serialized before making it a firearm. Other states have banned certain gun-making kits.
This guide has some important definitions and phrases that you should know, and it answers some questions you probably have. Not even sure what an 80% lower is? Start there.
Next, decide what you’ll build
Ask yourself, “What is my goal with this project?”
Do you want to come out of the dust wielding a top-tier tactical shooter, one that gives big brand guns a run for their money? Maybe you just want to spite the government and see how the process works, putting your name on a homemade gun you can say you built.
Or does your project fall somewhere in the middle? Picking the type of gun you want to build – and the overall goal of your build – will largely decide which 80 percent lower and tools you buy.
Available calibers and configurations
You can easily build your AR-15 as a rifle, SBR (short-barreled rifle) or a pistol, and the platform offers shooters a multitude of calibers to pick from: 6.5 Creedmoor, .308 Winchester, 5.56 NATO, .223 Remington, .300 AAC Blackout, .22 LR, 12 gauge shotgun rounds, are some of the most popular choices.
Caliber matters most of all
When it comes to buying an 80% lower, the only thing you truly need to care about is the caliber you’ll be shooting. Regardless of your AR’s configuration and setup, the parts you buy will install the same way.
There is nothing physically different about a completed 80% lower when compared to a store-bought lower receiver. Both look and function the same way, and both use the same parts depending on what type of gun you’re building.
Available Receiver Types
Now that you have a warm-and-fuzzy about the legality of this project, it’s time to dive into the meat.
There are four types of receiver materials to choose from: Forged, billet, cast, and polymer.
Take your time getting familiar with the different types of receivers. Next, we’re going to look at all the available options and which type of receiver is best for each type of build.
Forged 80% lower receiver
Ah, the O.G. unit. The forged lower is the most popular, most affordable, most rugged, and so on. It’s the #1 choice among builders and it’s what the military uses. It’s capable of handling heavier cartridges and taking a beating thanks to its high tensile and shear strength.
Pros and cons
Forged receivers are harder and denser than any other type. They will eat up your drill bits and end mill bits more quickly than other types. Drilling will also take longer and you’ll need to make sure your bits aren’t walking or misaligned.
With that said, experienced machinists say cutting forged aluminum is actually easier than cutting billet. The denser material cuts more slowly, but it also provides a cleaner, more controllable cut with smaller shavings and debris.
Billet 80% lower
The billet option is a step up from the forged receiver in terms of features and perks. Because billet receivers can be cut into more complex shapes, they get the benefit of having an integrated trigger guard and pre-threaded holes for some of the lower parts kit pieces. These threaded holes replace the original roll pins used in standard kits. This makes final assembly much easier.
Pros and cons
Billet receivers look cool and they take less of a toll on your jig and tooling. But like we just said, they may actually be more difficult to cut and drill than a forged unit. Billet is softer than forged. The softer metal is more prone to suffering damage from small mistakes while you cut and drill.
Polymer Lower Receiver
The new alternative to a forged or billet unit, a polymer lower is a great option if you want to build an incredibly lightweight AR-15. These polymer receivers weigh half as much as a metal receiver. Because they’re made from polymer, these receivers are incredibly easy to drill and cut and they include their own jig and tooling.
Pros and cons
Polymer is slightly less rugged than billet or forged aluminum. A polymer receiver is more prone to suffering light scratches and blemishes on the surface. Any tactical rifle or pistol will eventually suffer the same markings from regular use, so this shouldn’t be taken into consideration.
Again, cutting and drilling are the most important factors to consider. Because you’re working with a polymer receiver and jig, you’ll need to be careful and make sure you don’t remove too much material or accidentally damage the jig itself. Luckily, the jig is disposable and since you can use a simple pair of snips and hand files to do much of the work, the risk of making a mistake is very low.
Other Things to Consider
(Polymer builders can skip this, your receiver doesn’t require a finish) Once you’ve selected your type of lower, you’ll need to decide: Do you want to customize its look and apply your own protective coating? Or, would you rather invest in a good ole’ mil-spec finish?
The latter is the default choice for most builders. Both billet and forged lowers can be grabbed with a Type III Hardcoat anodizing that seals the aluminum underneath from the elements. This is the same coating that service-issued M4s and M16s get. Of course, applying your own coating will mean you’ll need to buy a raw lower, one with no finish applied (but you’ll save a couple bucks).
Under federal law, a homemade firearm isn’t legally required to have any serial number or other engravings (though the ATF recommends it on page 8, should your firearm ever get lost or stolen).
The same doesn’t always hold true for state law. California requires you to submit an application for a unique serial number that you’ll need to get engraved on your receiver blank before you cut and drill it. Just double-check your local laws before you start working on your 80% lower.
As for other engravings, it’s important to have “FIRE” and “SAFE” engraved for your safety selector lever, if you want to practice correct firearm safety. Most lowers have these markings pre-engraved, save for raw (un-coated) units.
Once you’ve finished your receiver blank, it’ll be time to install your lower parts kit – you’re now officially building your AR-15! Certain receivers have different fittings and assembly methods. For example, forged receivers use roll pins to secure things like the bolt catch and magazine release. Billet receivers (which can be cut into complex shapes) will often have those same fittings pre-threaded for screws.
Threading in some small screws is a helluva lot easier than fidgeting with roll pins and tiny little punches. If you’re a first-time builder and want to master the practice, stick with forged. If you want assembly to be easier and quicker (no one’s faulting you), stick with billet.
Gear! An 80 percent lower’s really just an excuse to buy some cool power tools and shiny bits. Whether you buy a polymer or aluminum receiver, you’ll need one type of drill and one type of powered cutting tool:
- 80% lower jig
- Handheld drill
- Drill press (good)
- Handheld router (ideal)
- Milling machine (ideal)
- Tabletop vise
Cutting Your Receiver
No matter the lower, you can easily drill the hammer, trigger, and safety pin holes using a handheld drill. Cutting the fire control group cavity (the inside of the receiver where the lower parts kit and trigger install), will be easiest with a mill. You can cut out the cavity on an aluminum receiver with a drill press and the provided end mill bit using plunge cuts.
Drill press vs. mill
Experienced machinists have had good results milling their receivers using a drill press. This takes time and patience since drill presses aren’t designed to cut side-to-side like a milling machine.
Doing any cutting with a drill press like this is made much easier by first drilling vertical pilot holes. Most jigs include a top plate with a pilot hole template to do just this (pictured above). This removes the bulk of the aluminum in the cavity using drill bits, leaving small shavings and burrs that can be easily plunge-cut or carefully milled without much resistance.
With your 80 percent lower cut and drilled, coated or customized, you’ve officially built your first unregistered firearm. You’re now the proud owner of an AR-15 lower receiver! All that’s left is to throw in a parts kit and trigger, slap on an upper receiver and buffer system, and hit the range.
If you need help ordering a receiver, have more questions, or simply need help building, give us a call or email us!