How To Complete 80% Lower Receivers | 5D Tactical thumbnail image

How To Complete 80% Lower Receivers | 5D Tactical

5D Tactical - 29th Apr 2022

In recent years, there has been increasing interest in building an AR-15, AR-308, or AR-9 in the comfort of your own workshop. That’s why many home smiths invest in 80% lower receivers.

Milling out these incomplete components enable you to construct your own firearm, and instill a better understanding of how these machines work. However, to get the most out of your own mill, it’s advantageous to refer to step-by-step instructions for guidance.

Fortunately, at 5D Tactical, we have the expertise and collaborative spirit to empower you to build up the armory of your dreams. This is our comprehensive guide on how to complete 80 percent lower receivers.

How to Build 80% Lowers: Complete Guide

Here, we’ll list out the essential tools, teach you how to mill out and finish your lower, and address any concerns you may have in pursuing this new hobby. Before commencing your project, we recommend that you read the article to the end, and write out a list of the specific gear that you require.

After acquiring everything you need, it’s advisable to print out the step-by-step guide and have it handy during the process. This reduces the likelihood of getting oil on a computer or mobile device, and minimizes downtime from unlocking screens and recharging batteries.

Another thing we like to recommend is that you keep a pen and notebook handy as well. This way, you can take notes that enable you to perfect your skills as a DIY smith.

Tools Needed to Complete 80% Lower Receiver

To complete an 80 lower, you’ll need the following tools and supplies:

  • 80% lower(s)
  • 80% lower jig i.e. Router Jig Pro
  • End Mill that’s compatible with your router
  • Vise
  • Cutting Fluid or WD-40
  • Shop Vacuum
  • Screwdriver Set
  • Allen Key Set
  • Eye Protection
  • Ear Protection
  • Gloves

How Long Does It Take to Complete an 80 Percent Lower Receiver?

Finishing an 80 lower can take up to about 4 hours for a novice. Some of our experienced customers have reported completing their 80 percent lower in under an hour!

80 Lower Milling Instructions: Step-by-Step

Now that you’ve gathered all your tools and supplies, it’s time to get to work!

Follow these steps to ensure quality results on your 80-lower:

Step 1: Assemble 80 Lower Jig

Before you start any machining, make sure you put your jig together.

Loosely screw on the several parts comprising it to prevent any unwanted movement during the milling process.

(You’ll also want to have a vise handy soon.)

Step 2: Install Lower Receiver

If you’re using our Router Jig Pro, you’ll be able to install any AR-9, AR-10, or AR-15 80 percent lower receiver on the market.

  1. Apply the buffer plate to the back of the receiver.
  2. Loosely thread the buffer adapter into the receiver until the buffer plate is firmly pressed against the back of the receiver.
  3. Loosely install two jig screws to secure it to the guide plate (the shiny blue top). Next, you’ll want to loosely install the takedown adapter to the guide plate with two jig screws.
  4. Align the 80 percent lower receiver with the adapter
  5. Use the caliber-specific pin to secure the lower to the jig
    • The buffer adapter also doubles as a hose attachment, which is perfect for shop vacuums.
  6. Screw on the guide for the lower receiver’s pilot hole.
Now you’re ready to install the left and right sidewalls.

There are a total of eight jig screws, make sure they are loosely threaded on at first and then tighten afterwards.

Once all that’s been done, mount your jig to a vise before you start any drilling or milling.

Router Jig Pro

Step 3: Install Router Plate Adapter

Now we have to get the router ready.

  1. Install the router plate adapter to your router with the #1 (short) guide pin screws by using a 7/64” allen wrench.
    • The open end of the pins should be facing up. Do not over tighten.
  2. Make sure the pins are fully seated
    • Keep in mind: these pins will be changed out throughout the milling process.

Step 4: Drill Out Pilot Hole

Now that the router is ready and the 80 lower is mounted to the jig, install the drill guide with two jig screws.

  1. Spray WD-40 into the drill guide hole and insert the 21/64” drill bit.
    • Only start drilling when the bit is fully inserted.
  2. Carefully drill straight down and perpendicularly to the lower receiver.
  3. Frequently pull the drill bit out of the guide to clear away metal shavings and chips.
  4. Re-apply WD-40 as needed and liberally.
  5. Drill until the bit exits the bottom of the receiver.

Step 5: Begin Milling Process

  1. Set the end mill depth to the first hash mark using depth gauge #1.
    • Be sure the guide pins are not between the adapted and guide plate.
    • Make sure the router depth adjustment is locked after each depth setting is changed.
    • Per the guide pins size (#1, #2, or #3), they should be positioned in their respective cavities on the top guide plate which are also labeled #1, #2, or #3.

Step 6: Ensure Trigger Slot Cutout

As you reach the final depth hash mark, you’ll start to see the slot cutout more defined for your trigger.

  1. Remove the #2 (medium) guide pins
  2. Install the #3 (long) guide pins
  3. Set the end mill to the bottom of the #3 depth gauge
  4. Center the end mill where the initial pilot hole was drilled
  5. Gently mill in a clockwise direction until the trigger slot is fully formed

With certain routers, the spindle may not always be able to fully reach the bottom of the cavity. Be sure to check your tools against our router compatibility chart and ReadyMill sizing guide.

Step 7: Drill Out Hammer and Safety Pin Holes

These are the final drilling steps, in which you’ll need to put down the router and pick up your power drill one last time.

  1. Adjust the entire jig to be clamped on the edges of the side plates with the RIGHT side wall facing upwards.
  2. Ensure the assembly is level.
  3. Spray WD-40 in to the large guide hole and insert a ⅜” drill bit.
  4. Only start drilling when the bit is fully inserted in the guide hole.
  5. Apply moderate pressure and drill until the bit penetrates all the way through to the left side wall.
  6. Repeat the same steps for the 5/32” drill bit into the two remaining guide holes.

Common 80 Lower Mistakes to Avoid

Do not forget to wear eye pro and ear protection as it can get a bit loud and metal shavings will fly everywhere. No one wants to lose an eye like this.

Gloves may not always be necessary but the metal shavings can be hot and sharp. Having gloves may increase your confidence and work speed.

Wearing a mask can also help with the burning smell that periodically arises.

Don’t Rush

It’s easy to want to rush through the process but it’s much better to take your time. This helps you to avoid damaging the walls of the pocket that you’re working so hard to mill out.

One of the most common mistakes that people often make is skipping hash marks on the depth gauge that is laser engraved on the top guide plate. Our 5D Tactical Router Jig Pro User Manual details all of this.

One might think that this is like taking a shortcut, but in reality, you are just reducing the lifespan of your end mill and increasing the chance of it breaking. You’ll know if you’ve exceeded the length of the router’s reach if you experience some “chatter.”

What is Router Chatter?

Chatter will come in the form of violent vibrations and if you don’t have a good grip on the router it could even bounce around within the lower receiver, damaging the internal walls.

Use plenty of cutting fluid!

WD-40 has worked great in our experience, you really don’t need to buy anything fancy if you don’t want to. We recommend spraying WD-40 after every pass to ensure lubricity throughout the entire milling process.

Complete Your 80 Lower and Your Armory with 5D Tactical

Building your own rifles, carbines, and Glock-style pistols can be rewarding, fulfilling, and proactive. Not only are you properly armed to protect yourself, your family, and your property, but you’re also equipped with the knowledge to stay fortified.

By staying stocked up on lower receivers, and keeping your mill in operation, you won’t have to rely on your local firearms shop to keep you armed. Best of all, building your own rifle is easier than ever with our step-by-step guide.

At 5D Tactical, we’re passionate about spreading the DIY smithing hobby. If you want to speak with an expert about your next project, reach out to our team today.