Is it difficult to machine an 80% lower? thumbnail image

Is it difficult to machine an 80% lower?

80 Percent Arms   |   Jun 4th 2020

What is involved in machining an 80% firearm?

So you've finally decided to pull the trigger. You've been contemplating it, debating with yourself about it, and you've finally convinced your wife to let you build an AR-15. Yet, with the capacious number of options when it comes to the Barbie of firearms, you're still on the fence about how to approach it. One such option is the 80% lower build.

What’s an 80% lower you may ask? Quite simply, an 80% lower is an incomplete lower receiver which requires additional tooling before it can be used as a firearm. Therefore, the ATF does not recognize 80% lowers as firearms. What does this mean for you? It means you don’t have to order your lower through an FFL. This cuts out the middleman to start building your AR. You ship it straight to your house and start cutting away the additional 20% of metal needed to insert that cool trigger group you’ve been drooling over. But that’s for gunsmiths, right? Well, yes, but also no.

Although the process requires machining, it can be done with fairly simple tools that don’t cost a lot by themselves. In fact, some of these tools are probably collecting dust in your toolbox right now. However, if you don’t happen to have any of the required tools, it could cost you around $500 to acquire them. This brings us to one concern to consider before diving head first into an 80% build. You should determine if this will be a one-time deal or if you are planning to build more than one AR. If you just want to build a one-time project firearm, then perhaps purchasing a complete lower is best for you, especially if you don’t possess the tools needed.

On the other hand, if money is no concern, you have all or some of the required tools, or you plan on building multiple ARs, then getting into 80% lowers may be right up your alley. Assuming this is true, your next question may be, “Is it difficult to machine an 80% lower?” That is what this article hopes to answer. Ultimately, it depends on the person. Everyone is unique with different skill sets. So the point of this piece is to give a general overview of the process and potential difficulties that may accompany it.

First and foremost, we need the tools. You know, the tools we’ve been alluding to for the last three paragraphs. These tools can be found at your local hardware store, but if you are unsure about what you should get, there are several online locations that sell these tools specifically for the purpose of completing 80% lowers. Here is the list of what you need:

  1. AR-15 Jig
  2. Hand drill or drill press Note: If you are using a hand drill, be sure to use a vise.
  3. Router
  4. ⅜ Drill Bit
  5. 19/64 Drill Bit
  6. 5/32 Drill Bit
  7. End mill

Other items you may want to have on hand are cutting lubricant, compressed air (either in a can or via a hose), painter’s tape, and, of course, ear and eye protection. Now that we have all the tools needed to start your first 80% AR, let’s get started.

There are plenty of “How-to” articles floating among the umbral edges of the internet. Therefore, we will not go into depth regarding the process. We will, however, talk about key points and tips. To begin with, be sure you have an AR-15 Jig. A jig is simply a guide or template similar to a stencil. It allows you to make the exact cuts needed to complete your AR. However, you must take care to read any instructions provided so as not to ruin your lower. The last thing you want is to waste time and money on something that could have been avoided.

Next, follow instructions on attaching your jig to the AR lower. There will be cutouts within the jig that will guide your drill and router in the way they should go. The best tip with cutting into the lower is to drill into the metal little by little and to remove excess aluminum shavings as often as possible. Additionally, be sure to lube your drill bits before each use. Doing so will ensure a longer life for your bits and an easier cutting experience. Take your time, especially if this is your first attempt.

Lastly, drill your safety selector holes, trigger/hammer pin holes, and trigger slot pilot hole after you’ve cut your trigger pocket. This step may differ person to person depending on their machining experience. For those who are new to the process, it may be best to do this step after the main cuts. Also, be sure not to cut straight through to the opposite side when cutting these holes (again, user experience applies). Once these are drilled, you are now complete. Congratulations! You’ve finished your first 80% build! You may now adorn your new lower with all the cool AR parts you’ve secretly accumulated in your closet.

Now that you’ve finished your lower, let’s get back to the point of this article. How hard was it? Generally speaking, drilling out your own AR is pretty straight forward. Pretty much anyone with general knowledge of the AR platform can accomplish it within 5 hours, often less. Once you get enough practice, you’ll be knocking out AR lowers quicker than Jerry Miculek’s trigger finger. Just kidding. No one is that fast. The point is, the process may seem daunting at first, but it is completely doable, even with novices. There may be a learning curve, but nothing that can’t be overcome.

Building an AR is already fun as it is, but there is something uniquely grand about machining your own firearm. This is the great appeal of building an 80% lower. You can actually say you “built” your AR, more so than the average AR enthusiast. Every time you look at your build you can have the pride in saying, “I did that.” So what are you waiting for? Go out there and build that AR!