This is our comprehensive guide on how to complete 80 percent lower receivers. We’ll teach you how to mill out and finish your lower, list out the essential tools you’ll need and address any concerns you may have in pursuing this new hobby.

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 To Build 80% Lowers: Complete Guide

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

80% lower jig router jig pro 5D tactical

Before you start any machining make sure you put your jig together and loosely screw on the several parts 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. First, apply the buffer plate to the back of the receiver. Loosely thread the buffer adapter into the receiver until the buffer plate is firmly pressed against the back of the receiver. 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. Once installed align the 80 percent lower receiver with the adapter and use the caliber specific pin to secure the lower to the jig. 

You can also double the buffer adapter as a hose attachment which is perfect for shop vacuums. Next, 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. 

Step 3: Install Router Plate Adapter

router plate adapter

Now we have to get the router ready. 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 overtighten. Make sure the pins are fully seated and know that these pins will be changed out throughout the milling process.

Step 4: Drill Out Pilot Hole

pilot hole drill guide

Now that the router is ready and the 80 lower is mounted to the jig, install the drill guide with two jig screws. Spray WD-40 into the drill guide hole and insert the 21/64” drill bit. Only start drilling when the bit is fully inserting. Carefully drill straight down and perpendicularly to the lower receiver. Frequently pull the drill bit out of the guide to clear away metal shavings and chips. Re-apply WD-40 as needed and liberally. Drill until the bit exits the bottom of the receiver. 

Step 5: Begin Milling Process

milling out 80 lower

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. Remove the #2 (medium) guide pins and install the #3 (long) guide pins. Set the end mill to the bottom of the #3 depth gauge. Center the end mill where the initial pilot hole was drilled and 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 so be sure to check your tools against our router compatibility chart and ReadyMill sizing guide

Step 7: Drill Out Hammer and Safety Pin Holes

drilling out hammer and safety pin holes

These are the final drilling steps where you’ll need to put down the router and pick up your power drill one last time. Adjust the entire jig to be clamped on the edges of the side plates with the RIGHT side wall facing upwards. Ensure the assembly is level. Spray WD-40 in to the large guide hole and insert a ⅜” drill bit. Only start drilling when the bit is fully inserted in the guide hole. Apply moderate pressure and drill until the bit penetrates all the way through to the left side wall. Repeat the same steps for the 5/32” drill bit into the two remaining guide holes. 

How Long Does it Take to Complete an 80 percent lower receiver?

completed 80% 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! 

Common Milling Mistakes to Avoid

It’s easy to want to rush through the process but it’s much better to take your time so you avoid damaging the walls of the pocket that you’re working so hard to mill out. One of the most common mistakes that people will 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 this.

One might think that this is like taking a shortcut but in reality you are just reducing the lifespan of your end mill, increasing the chance of it bre aking even. You’ll know if you’ve increased the length of the router’s reach by too much if you experience some “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. 

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 aren’t 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. 

Are 80 Percent Lower Receivers Worth it?

Are you worth it? If you’ve ever thought about taking the 2nd Amendment into your own hands, smash that subscribe button to our emails. There are a lot of benefits to investing in 80 percent lower receivers. Namely, being able to build a firearm that the government doesn’t know about. If you’re not a fan of government lists this project is perfect for you. Imagine if the pandemic worsens again or if there’s another wave of civil unrest — the gun stores will sell out again and rather than spend an entire day lining up at your local FFL that may not have any inventory available… you can skip the hassle by completing 80% lower receivers.

Why You Can and Should Build Your Own Firearms

custom ar-15 pistol build

Making your own firearm has been a longstanding and time honored American tradition that wasn’t talked about much until recently. As a part of the Second Amendment, building your own firearm is protected under the Constitution and is widely accepted in most states. It’s really only the coastal states and Illinois that have an issue with 80 lowers. 

Aside from the legal rights aspect, building your own firearm is also highly practical. It allows you to become that much more of a gun nerd. Instead of buying a factory built gun off the shelf, buying each individual component and putting it together gives you a more intimate understanding of how firearms work, what they can do and their limitations. Perhaps the best part, you can make it look as wild as you want!

The extra knowledge could give you a leg up in the next competition. It could give you the advantage in the event you have to defend yourself or others. So join us in our endeavor of normalizing this hobby, protecting this industry and taking back our 2A rights.