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 your receiver blank is where the 80% jig comes in. Where big gun makers use factories, industrial mills, and CNC machines, the at-home gunsmith like you uses the jig to replace all that big equipment while producing the same results.
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 metal guides, basic depth measurements, and hand tools. The jig acts like a template and uses special plates that show you where to cut away extra aluminum and where to drill holes in the receiver:
The 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. The jig we're using for this guide is the popular Elite Builder Jig.
How the Jig Works: An Exploded View
There are plenty of jig options available today, but they all generally use the same setup, as shown above. After all, the AR-15's lower receiver is a universal design, so the fabrication steps required to turn an 80% lower into a stripped lower must be uniform. There is only one correct way to cut and drill the receiver, so all jigs must essentially work the same way:
1. Left and Right Side Plates
The side plates secure the 80% lower receiver for machining. These side plates also contain the pin hole guides for drilling holes to install the hammer, trigger, disconnector, and safety selector lever pins:
2. Template Holder Top Plate
The fire control cavity (FCC) is the area within the lower receiver where you'll install the parts kit's components. Since the 80% lower's FCC is filled in with aluminum, it needs to be milled out. This template-holding top plate acts as a platform securing the various templates you'll use to mill the FCC:
3. Pilot Hole Template Plate
This is the first plate you'll use to get started on milling the FCC. It provides various guide holes for drilling pilot holes into the cavity. This removes a large portion of aluminum, and it creates negative space for inserting the end mill bits for cutting out the remaining aluminum. Some jigs only provide a single pilot hole and are designed for the end mill bit to do most of the work, but this can reduce the overall life expectancy of that more-expensive bit. Drilling multiple pilot holes is quicker and spares the life of your end mills.
4. & 5. Fire Control Cavity Templates
These templates are to be used in tandem with the end mill bits once your pilot holes have been drilled. Two templates are often required: One for completing the rear shelf area, and one for cutting the main cavity, which runs deeper into the floor of the receiver than the shelf. The rear shelf area contains additional aluminum to reinforce the receiver, and it also contains a threaded hole for securing the pistol grip:
6. Trigger Pocket Cutting Template
Last but not least, you need to cut the slot for the trigger to rest below the floor of the receiver, inside the trigger guard. The final template provides the exact shape required for cutting this slot with a smaller end mill bit:
7. Screws, Bolts, and Allen Keys for Securing Jig
Of course, the various jig plates and templates need to be secured together so they don't move while you're drilling and milling. A series of bolts, screws, and the Allen keys to tighten them are included with each jig.
Required Bits for Completing an 80% Lower
Again, since the AR-15's lower is universal in its design, the drill bits and end mill bits required for drilling and cutting the receiver must be uniform. There may be variances in end mill bit diameter for cutting the FCC but this is not common. The trigger slot end mill bit and drill bits are universal, to ensure the diameters of the trigger slot and pin holes are correct for all parts kits.
3/8" Short End Mill
This is the typical size end mill provided for cutting the main cavity. The short shank reduces bit flex when the bit is placed under load, reducing chatter and poor cuts. It also sports a relatively short length of cut because relatively shallow passes are made with each cut. The depth of the cut is increased in small increments until the final depth is achieved.
3/8" Long End Mill
This end mill provides the same typical cutting diameter, with a longer shank to accommodate the deeper required cut for completing the main portion of the FCC.
5/16" Long End Mill
A 5/16" end mill bit provides the correct cutting diameter for completing the trigger slot. It has the longest shank to ensure it is capable of fully cutting through the floor of the receiver.
5/32" Drill Bit
This bit provides the appropriate diameter for the trigger, hammer, and disconnector pin holes. It is a universally required drill bit for any jig and AR-15 lower receiver.
3/8" Drill Bit
This bit provides the correct diameter hole for the safety selector lever. It, too, is universally required across all AR-15 jigs and receivers. This bit is also most commonly provided as the pilot hole bit for the FCC, which is why most jigs and 80% lower kits come with 3/8" end mill bits. It's a simple matter of convenience to provide end mills that are the same diameter as this bit. This eliminates the need for a third drill bit of a different diameter for the pilot holes.
Types of Jigs: Router vs. Drill Press/Mill
There are two types of 80% jigs: Router jigs, and drill press or milling jigs, like the one shown in this guide. Like we said earlier, most jigs use the same parts, top plates and templates, 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 a router instead of a drill press or mill. Drill press/milling jigs ditch the handheld router and use a drill or milling machine to cut the fire control cavity.
Heavy-duty drill presses with an adequate chuck can do light milling with aluminum, though the end mill bits provided with drill press jigs are center-cut, which allows for a process known as "plunge-cutting". In this fashion, the end mill is used like a drill bit, cutting vertically to the appropriate depth in the receiver. This is a safer method for lightweight drill presses.
Which Jig is Better?
Either type of jig will yield the same results for your finished 80% lower receiver. Router jigs might work faster than a drill press because you can mill more quickly with a router than you can make plunge cuts with a drill press, but routers can be dangerous because the cutting implement is being controlled by hand. Most routers are designed for cutting wood, not aluminum alloy. The depth of the end mill bit must also be adjusted manually, since routers don't provide the same level of vertical control as a drill press or milling machine.
Advantages of a Drill/Mill Setup
Drill press and milling 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. With the bulk of the FCC completed, light polishing can be accomplished by milling in a traditional fashion with the press to remove any small burs or bumps on the receiver floor or walls.
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 like a drill press. The jig shows you where and how to machine the receiver blank, so little to no experience is required. Router jigs may be quicker than a drill press jig, but they require more care and safety precautions. Drill press jigs may be slower, but they're easier to control and more precise in their machining.
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!