How To Complete An 80% Lower Receiver

Apr 16th 2018

How To Complete An 80% Lower Receiver

Power tools, end mill bits, sharp aluminum... The thought of finishing your own 80 lower receiver may seem a little intimidating. Don't be worried. With a few simple tools, you can easily cut and drill your own receiver blank in a single day with no experience required. This guide includes 80 percent lower milling instructions, and it'll show you where to cut and drill your receiver blank. This will allow you to transform that hunk of raw aluminum into a stripped receiver (legally a firearm), putting you on the path to building your own, unregistered AR-15 at home from scratch!

New here? Find out what an 80% lower is first.

Completing An 80% Lower: The Tools

Step 1: First, you'll need to acquire some safety stuff, drill bits, and hand tools. High-quality end mills and drill bits will perform better than store-bought stuff. Most 80% lower jigs include all this tooling that you need, too. If you haven't picked up a jig with bits, grab one now.

These are the bits and tools you need:

  • 80% Jig
  • 5/32 "" Drill Bit
  • 5/16 "" Drill Bit
  • 3/8 "" Drill Bit
  • 3/8 "" End Mill
  • Machining oil
  • Painter's tape
  • Canned air and brush
  • Ear and eye protection

The jig we're using in this guide is a drill press- and router-based jig. This is a single-caliber jig, designed for 5.56/.223 lowers only. The same cutting and drilling steps can be applied to other jigs for LR-308 and AR-10 builds. This particular jig can be used with a drill press or a router and handheld drill. We did this because you'll be using one or the other, and we want to illustrate both methods.

You'll need some power tools, too:

  • A handheld router and drill, or
  • A tabletop drill press

Plus, either way you'll need...

  • A tabletop/drill press vise

If you have access to one, a proper milling machine will work best for this project. It'll offer the quickest results and cleanest cuts. If you don't have access to one, no worries. They're expensive and all 80% jigs were designed to be used with handheld drills, presses, and routers.

Now that you've got the jig and tools together, let's get started.

Get Familiar With The Jig

First, unpack your jig and take inventory of all the parts. Make sure you have the appropriate plates: Left and right side plates, router plate (if applicable), spacer plate (if applicable) and the drilling and cutting template plate(s):

For our jig, the LH and RH side plates are pictured in the top left. Underneath them is the router plate (bottom left). In the center, top-down, are all the template plates. First is the drilling template. Underneath, note the little white notches and cut-outs on both the cutting templates (center middle and center bottom).

These are your depth gauges. You'll be using these gauges to measure and set the length (and thus, the cutting depth) of your end mill bit in your drill press or router.

We also have a shiny spacer plate (center right). The spacer plate does exactly as the name says: It ensures there's enough space for your end mill bit to cut the lower, without going too deep. Your jig may not come with a spacer plate. Some multi-platform jigs also combine all the top plates into one, universal template and router plate. Lastly, in the bottom right, we have the bolts used for securing the jig and 80% lower together as one assembly.

Step 1: Secure the Lower and Jig.

To start this project, grab your 80% lower and masking tape. Cover the exterior with tape to protect the finish. Leave the top of the receiver blank and free of tape. The top plates need to sit perfectly flush. Even a little bit of tape could throw off the final measurements:

Step 2: Secure the first top plate.

Next, secure the first top plate. If you're using a drill press jig, then your first plate will be a drilling template like the one pictured below. The drilling template will be used to drill out much of the aluminum inside the fire control cavity - the pocket where you'll install your lower parts kit and trigger.

IMPORTANT: Some 80% lowers have the rear shelf (the area where the back of the upper receiver and rear take-down pin secures inside the lower) fabricated already. If that's the case, you should mask off this area with tape after installing the template. This will prevent you from accidentally drilling down into this area and damaging the jig's bolts and receiver.

Step 3: Set your first drill depth.

With the jig secured and the lower ready for drilling, it's time to set the depth of your first drill bit. Grab the 3/8"" drill bit and drill stop collar (if included). Also, grab the Rear Shelf Pocket plate (labelled ""MAIN""), if you're using the same jig, or one like ours. If not, grab the appropriate plate with the appropriate depth gauge. For this jig, the depth should max out by touching the bottom of the depth gauge. With the depth set, secure the stop collar to the shank of the drill bit using the provided Allen key and screws:

Step 4: Secure the assembly in the vise.

Now we need to secure the jig and 80% lower assembly in our vise. This will prevent the assembly from moving around while we cut and drill. It's very important the jig doesn't move at all. Even a tiny movement will throw off your positioning. This could result in your lower not working at all:

Step 5: Start drilling each pilot hole!

This can be completed with a handheld drill or drill press. We recommend using a press. If you use a hand drill, be sure to keep it vertically aligned and drill slowly, with plenty of lubricant.

Now the fun begins! You can start drilling the fire control cavity, removing the bulk of aluminum for the lower parts kit and trigger. Make sure the jig and lower are secure enough that they won't move, but not too tight in the vise. Lubricate the entire assembly with plenty of cutting fluid. Begin drilling through each hole on the template until you reach the stop collar. Repeat this step for all unmasked holes.

IMPORTANT: Be sure to clean out each hole after drilling. Note all the aluminum shavings below. These shavings can damage your bit, lower, and/or jig if they're not cleaned out. Use canned air and a brush to keep the work area clean.

IMPORTANT: Check your drill bit's depth after every. Single. Hole. Trust us. The stop collar likes to loosen up, and your collet might get loose, too. Just grab the depth gauge and confirm the depth is the same, to be safe.

Drill each hole until complete:

Once all the holes are drilled, your 80% lower should look like this:

*Step 6: Drill the rear shelf.

*You may not need to perform this step. Remember, some 80% lowers have the rear shelf completed for you. If your shelf is fabricated, skip to step 7.

If your rear shelf isn't completed, follow these steps: Loosen the top rear bolt securing the lower to the jig. Keep the bolt secured in the jig, and partially secured in the lower. This will prevent the lower from slipping in the jig:

Remove the masking tape from the rear shelf template holes. Locate the bit depth gauge labelled ""REAR"" and set the 3/8"" drill bit to the bottom of the ""REAR"" gauge:

Then, simply drill the holes for the rear shelf until completed:

Step 7: Set up the cutting templates.

Congratulations! Your fire control cavity is nearly finished. We've removed much of the aluminum with our drill press (or handheld drill). Now we can finish removing the rest by cutting it out with our end mill bit. Now, remove the drilling template from the jig. Grab the router plate, the spacer template, and the trigger pocket template, pictured below:

Now, install the spacer plate and the trigger pocket template on the jig:

And lastly, install the router base plate. This isn't necessary if you're using a drill press instead of a router:

Step 8: Install the end mill bit, set the depth.

Now, install the jig's included end mill bit in your router or drill press. Grab the ""MAIN"" depth gauge on the Rear Shelf Pocket template. Set the end mill's depth to the first notch:

Step 9: Mill the fire control cavity.

With your end mill secured and set to the right depth, start cutting the fire control cavity. If using a router, turn on the router and carefully lower the bit down into one of the pilot holes you drilled. Once the router's fully seated on the base plate, start slowly moving the router from hole to hole, cutting out the remaining aluminum.


A drill press isn't designed to make side-to-side cuts like a router or mill. Instead, you'll need to make plunge cuts: Turn on the press, spin up the bit, and slowly plunge the end mill down into the cavity. Plunge the bit down until reaching the stop collar/set depth. Raise the bit back up, move the jig and lower around to line up a new cut, and make another plunge cut.

Repeat this plunge-cut or milling step with your drill press or router until the entire cavity has been cut to the first measured depth:

Once you've finished your first cutting pass, turn off your tooling. Remove the bit and drill press or router from the jig. Clean out the aluminum debris with your canned air or brush. Loosen up the end mill bit and increase your bit's depth. We recommend increasing depth by one additional 1/4 notch:

Repeat this entire step by increasing bit depth and making more cutting passes until you've bottomed out the end mill bit in the depth gauge.

Your final cutting depth and finished cavity should look like this:

*Step 10: Prepare to Mill the Rear Shelf.

*If your lower's rear shelf is already completed, SKIP STEPS 10 and 11.

With the main fire control cavity cut, it's time to cut the rear shelf. This means swapping out the trigger pocket template for the rear shelf template. Install the rear shelf template atop the spacer plate. Orient the template so the longer hole is near the buffer tube, and the shorter hole is near the front takedown pin.

Finally, reinstall the router plate atop the new template:

*Step 11: Set The Cutting Depth. Cut the Rear Shelf.

WARNING: REMOVE THE TOP REAR JIG BOLT that goes through the rear shelf area!

Now you can set the initial cutting depth for the end mill bit to complete the rear shelf. Grab the trigger pocket template and use the depth gauge labelled ""REAR"" to set the depth:

Repeat step 9 in the same fashion, adjusting your cutting depth by 1/4 notches, until bottoming out the mill in the depth gauge:

Step 12: Drill the Trigger Slot Pilot Hole.

Now you can remove all the top plates on the jig, including the spacer plate. Reinstall the trigger pocket template directly atop the jig side walls. Ensure the depth gauge on the template is facing down.

IMPORTANT: Attach the trigger pocket template to the jig side plates, inserting only 2 of the short bolts into the two holes on the template closest to the buffer tube. Use the two jig wall screw holes furthest away from the buffer tube to secure the template.

Finally, use the 19/64"" drill bit to drill the pilot hole for the trigger slot. Go all the way through the lower, but be careful not to drill the jig or trigger guard underneath:

Step 13: Reinstall the Rear Shelf Template. Cut the Trigger Slot.

Remove the trigger pocket template and install the rear shelf template directly atop the jig side walls. Ensure the raised lip on the template is facing up. Orient the plate so the larger hole is above the magazine well. This hole will not be used. Since we're cutting all the way through the floor of the receiver, a depth gauge is not required when setting up the end mill bit.

Adjust the length of the end mill bit so that the tip just rests inside the pilot hole you just drilled. Repeat step 9, making cutting passes and increasing depth, until you've completely cut out the trigger slot in the floor of the lower:

Step 14: Secure the Jig Horizontally. Drill the Pin Holes.

Congratulations! All the cutting requirements for this project are complete. Your trigger pocket template, trigger slot, and rear shelf are all ready for a lower parts kit. Now you just need to drill the trigger, hammer, and safety selector lever pin holes. The side walls on the jig have pre-drilled pilot holes that show you where you need to drill.

IMPORTANT: DO NOT ATTEMPT to drill the pin holes by drilling completely through one side of the jig and receiver. The drill bits will flex, and the holes will be mis-aligned. This will ruin the lower and prevent you from installing your parts kit. Each hole is marked with which drill bit you need to use:

You Just Completed an 80 Lower Receiver!

So, did you think completing your own 80 lower receiver was as difficult as it would be? You now also own the tools and equipment to build as many as you want! With an afternoon of free time, you can now skip the gun store check-out fees and extra taxes. Any time you're itching to buy a new black rifle, just pick up a lower and grab a parts kit.

Did you have questions about drilling or machining your lower? Need help with instructions or anything in this guide? Just give us a call or email us. We also build rifles and finishing lowers in our spare time. We're shooters, too!

DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.