Many black rifle enthusiasts and gun owners build AR-platform rifles and pistols without ever visiting a gun store or FFL. The whole process starts with buying an 80% lower, a receiver blank the end user fabricates to make a firearm “from scratch”. Receiver blanks aren’t considered firearms by the ATF until you machine them, so you can legally order and ship one straight to your house. Once you get your 80% lower or receiver blank, you get to turn it into a functional receiver using an 80 lower jig. Then, you can complete your build. If you’re here, you probably have some questions about AR-15 jigs, lowers, and how everything works. So, let’s break it all down.
Don’t know what an 80% lower is? Check out the complete guide here.
What’s an 80 lower jig?
An 80 lower jig, or simply 80% jig, is a small, portable workstation used to drill and cut an 80% lower, receiver blank, or frame blank, turning it into a firearm by legal definition. Most jigs are used to complete AR-platform lowers, but there are jigs now available for Glock and other handgun frames.
Most jigs use aluminum or steel side plates, top plates, an adapter plate for your power tools, and some long pins to secure your lower in place, like you see just to the right. Once secured, the lower gets drilled and cut by you, the lucky builder.
Some jigs come with multiple top plates, one for completing each machining step. Others use a single top plate for cutting the fire control cavity, the area inside your finished receiver that holds your trigger, hammer, parts kit, and springs. The type of jig you use will determine how many steps you must take to complete your lower, and which tools you’ll use.
How an 80% jig works
Even though we’re working with firearms, power tools, and metal equipment, using an 80% jig is safe and easy. If you’ve ever done any basic handiwork around the house or used a dremel, router, or hand drill, then you can probably build an AR or other gun at home. Here are the steps that you’ll complete on your 80% frame or 80% lower using a jig, depending on the gun you’re building:
AR-15, AR9, or LR-308 80% lower:
- Drill the hammer pin hole
- Drill the trigger pin hole
- Drill the safety selector lever hole
- Cut out the fire control cavity
Glock handgun 80% frame:
- Drill the trigger pin hole
- Drill the locking block pin hole
- Drill the trigger housing pin hole
- Cut the slide rails
- Cut the barrel seat
Types of Jigs (Drill Press vs. Router)
These little gunsmithing stations can be classified into two types: Drill press jigs and router jigs. Both look and function the same way, performing all the machining steps listed above. As the names imply, you’ll want a drill press jig if you already own a drill press, and you’ll want a router jig if you already own a handheld router and hand drill.
Router jigs are the more common choice among builders simply because it’s easier to go to the local hardware store and buy the tools you need. A router jig can work with a drill press as well, but not the other way around. However, drill press jigs are often more affordable and they provide better machining results.
They’re also more accurate and precise, and they require drilling fewer pilot holes (usually just one) for the fire control cavity before you begin cutting with your end mill bit. Router jigs cut more slowly, usually requiring multiple pilot holes to be drilled first.
Additional tools required
Most jigs come with all the bits and extra tooling you need (minus the router or drill press). Here’s what most AR-15 jigs come with by default:
- 3/8″ drill bit
- 5/32″ drill bit
- 21/64″ drill bit
- 5/16″ end mill bit
We recommend picking up some extra tools and bits to make using your 80 lower jig easy:
- Masking tape to protect your finish
- Machining oil or cutting lubricant (do not use WD-40)
- Drill press or tabletop vise for securing your jig and lower
Example of an 80 Lower Jig in Action
Let’s see how this whole process actually works. The jig used in this example is the Easy Jig Gen 2. The builder is completing a typical forged AR-15 80% lower for a rifle build. Here’s a quick-n-dirty summary, with pictures, of how drilling and cutting with a jig works:
Step 1: Secure your lower or frame
You’ll need to attach the side plates and top plate to your 80% lower or frame blank. The lower in this example is covered in masking tape. This will protect the anodized finish from aluminum shavings, debris, and scratches while you work.
Each jig might secure your lower differently, though most use a set of pins and the pre-drilled already made at the factory. Once your lower and jig are set up, you’ll need to secure the whole assembly using a tabletop or drill press vise. This will prevent the assembly from shifting, causing irreparable damage to your lower or jig.
Step 2: Drill the fire control pilot hole
It’s a good idea to drill and cut out the fire control cavity before you drill the hammer, trigger, and safety selector holes. Cutting the cavity first means you’ll have to drill through less material when you do drill those final holes.
We see here that the jig is properly secure to a drill press using a vise. The jig must be leveled and perfectly flat to ensure an even cut. Once secured and leveled, the pilot hole can be drilled.
In this case, the top plate uses a hardened drill bit stabilizer to pre-measure and align your hole for you. All you need to do is set the depth of your drill but. Setting the depth is done using a depth gauge included with the jig’s plates.
Step 3: Mill the fire control cavity
With the pilot hole drilled, you can swap out your drill bit for the end mill bit. The end mill bit will work alongside your jig’s top plate, cutting out the entire fire control cavity for you. You’ll need to remove material in a series of passes.
Again, the provided depth gauge, pictured to the right, will help you figure out how deep you need to set your bit with each cutting pass.
This is the only step where a router jig and a drill press jig differ. Cutting with an end mill bit can be completed with either tool. Also pictured is a handheld router being used for cutting. The elongated hole in the template (circled) is the cutting template, measured and shaped for you. The end mill bit’s shank or router plate (also circled) rides around the edges of that hole.
Step 4: Drill the hammer, trigger, and safety holes
With the fire control cavity milled out completely, all that’s left to do is drilling the hammer, trigger, and safety selector lever holes. These holes are pre-templated on your jig’s side plates.
Most jig plates include reinforced drilling templates or high-speed bearings to aid in precisely drilling without any friction or wear on the jig itself. With proper care and execution, most jigs are reusable. In this example, the side plates are labelled based on the caliber or platform you’re building. AR-15, AR9, and LR-308 lowers will have different templates and dimensions.
If your 80 lower jig is “multi-platform”, then it will likely be capable of completing all three types of lowers. You’ll need to follow the included instructions to ensure your jig and lower are set up appropriately. Other than that, the machining steps are the same across the board.
You should now have all the info you need about 80% jigs! Here’s a quick summary of everything, in case you were too busy to read through everything:
- An 80 lower jig is a tool used to complete an 80% lower or receiver blank
- Most jigs are compatible with the AR-15 receiver or multiple platforms (like LR-308)
- You can even buy jigs and frame blanks for completing a 1911 or Glock at home
- Jigs typically use two types of tools: A drill press or handheld router
- Router jigs are more convenient, but drill press jigs yield better resuls
- If you’re unfamiliar with building a gun at home using a receiver blank, learn what an 80% lower is first.