If you researched how to build an AR-15 from scratch, you’ve probably come across the terms “80 lower receiver” or “80% lower”. These are receiver blanks that you can use to build your own black rifle at home.
If you use a receiver blank to build your rifle or pistol, you don’t need to visit a gun store, deal with an FFL, fill out any paperwork, or pay any extra taxes or fees to the government (state laws may vary). If you’re still unsure what receiver blanks are, we recommend you read up on them first. This quick guide explains 80% lowers in detail.
What is an 80% Lower Jig?
Finishing the receiver blank is where the 80% jig comes in. Where big gun makers use factories, mills, and CNC machines, the at-home gunsmith like you uses the jig:
Gun makers have digital blueprints and pre-programmed tools that cut and drill their firearms at the factory. For you, the jig does the same thing using simple measurements and hand tools. It acts like a template and uses special plates that show you where to cut away extra aluminum and where to drill holes in the blank.
The whole goal of using a jig is to turn your 80% lower (receiver blank) into a stripped lower receiver. Note the red arrows indicating the drilled holes and finished fire control cavity:
Next, let’s break down the jig and inspect each part. We want to know what each part does, and how the whole thing works together:
How the Jig Works: An Exploded View
Below is an exploded parts diagram of the jig exampled above. It’s called the Easy Jig, and it’s one of the first jigs to hit the market years ago. The Easy Jig can function as a router or drill press jig (we’ll touch on that later).
There are plenty of jig options available today, but they all generally use the same setup. After all, the finished AR-15 receiver is a universal design. There is only one correct way to cut and drill it, so all jigs must essentially work the same way.
Let’s break it all down piece by piece:
Left and Right Jig Walls
Think of these jig walls as the foundation of the whole jig. The walls secure the 80% lower in place using the bolts pictured to the left, above. The side walls have pre-drilled and threaded holes on top (pictured below) for securing the different top plates/cut-and-drill templates. Most jigs secure the lower in place using the pre-drilled front and rear takedown holes:
The side walls double as drill templates, too. They show you where to drill the holes for the hammer, trigger, and safety selector lever pin holes on either side of the receiver blank:
This is the first template you must use. It shows you where to drill numerous pilot holes for completing the fire control cavity. This is the area where you’ll install your lower parts kit and trigger. Some drill templates only use one pilot hole. This one is designed to remove most of the metal quickly and efficiently with a drill press, eliminating much of the cutting work that comes later:
Spacer, Rear Shelf, and Trigger Pocket Template
The spacer is just a piece of aluminum that helps set the correct depth for your end mill bit. Cutting too deep will ruin the receiver. Atop the spacer is the first cutting template for the trigger pocket. Not pictured is the rear shelf template, which works the same way. With pilot holes drilled, the cutting templates let you mill out the rest of the aluminum with an end mill bit and handheld router:
Router Base Plate
The router base plate is a flat, smooth surface that connects tool to jig. This plate allows you to run your router atop the templates and cut the receiver without directly touching the jig itself:
Typical Jig Tools
Most 80 lower jigs come with a set of commercial drill bits and at least one end mill bit. Again, the AR-15’s receiver is universal. Most jig tool kits include the same set of bits, in order of appearance:
- Drill stop (for setting the bit depth)
- 5/32″ drill bit (hammer and trigger pin holes)
- 19/64″ drill bit (pilot hole for end mill, cutting template)
- 1/4″ end mill bit (rear shelf, trigger pocket template)
- 3/8″ drill bit (safety pin hole, pilot hole template)
Types of Jigs: Router vs. Drill Press
There are two types of 80% jigs: Router jigs, like the one above, and drill press jigs. Like we said earlier, both jigs use the same parts, top plates and templates (except the router base plate), side walls, and tools.
The only difference between the two is how you’ll cut the fire control cavity for the parts kit (and therefore, how those templates are designed). Router jigs work by letting you “mill” the receiver front-to-back and left-to-right using your router and end mill bit.
Drill press jigs let you cut the fire control cavity vertically with the end mill bit and a drill press, just like you would if you were drilling pilot holes with a drill bit. Machinists call this “plunge cutting”.
Which Jig is Better?
No jig is truly better than the other. Either type of jig will yield the same results. Router jigs might work faster because you can mill more quickly with a router than you can make plunge cuts with a drill press. Milling the fire control pocket with a router means you must first drill a pilot hole (or holes), then insert the router and end mill bit into the pilot hole(s) while the tool is spinning. This is often where mistakes (and possible injuries) happen.
Drill press jigs are more stable and precise, especially when comparing plunge cuts to handheld milling.
Using a drill press keeps your hands free of moving tools. It also allows you to slowly lower the end mill bit into the receiver, cutting with better control and precision. Pairing your drill press and jig with a cross-slide vise will make the work go even quicker and more easily. With a slide vise, just make one plunge cut and then move the jig and receiver using the vise, lining up the next vertical cut. Simple.
The 80% jig is a small workstation that allows you to cut and drill an 80% lower (a receiver blank that isn’t legally considered a firearm), to completion. Using an 80% jig requires basic hand tools: Either a router and hand drill, or a drill press. The jig shows you where machine the blank, so little to no experience is required. Router jigs are faster but require more care. Drill press jigs are slower, but easier and more precise.
Have more questions about jigs, lowers, or AR-15s? Give us a call or email us! We’re glad to help. We build receivers and black rifles in our spare time. That’s why we’re in this business!