How to Assemble a Lower Receiver

Introduction

An AR-15 Rifle build should start with purchasing a lower because this is the part that is an FFL item and requires a background check like any other firearm. Building an AR lower can look a little bit intimidating for a first-timer. But, with a little preparation and some guides, it’s more than doable. Here, we’ll walk you through the process in brief. First, it’s important to have the right parts and tools for the whole build. From there, we’ll talk about the magazine and bolt catches, before moving onto the trigger assembly. Lastly, we’ll get it ready to meet with an upper by getting the grip and buffer assemblies good to go.

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Get The Parts and Tools

To assemble a lower receiver, you’ll need the following parts, assuming you’re going with a build that aims at being simple:

  1. A stripped lower receiver (just the aluminum, no other parts).
  2. A lower parts kit (this will have most of the parts, some do not include a trigger, which you might want to get separately)
  3. A buffer tube assembly
  4. A Stock
  5. A pistol grip (or another state-compliant device)

While it is possible to buy all of the springs, catches, and screws separately, we don’t recommend this. Because there are so many, it’s entirely possible that you’ll forget something that will stop your build. Instead, it’s worth paying a small premium to simply have all of the parts you need from the get-go.

In terms of tools, we recommend:

  1. A Set of punches
  2. A Hex Key Set 
  3. A hard rubber mallet
  4. A C clamp or magazine well block in a vice. 
  5. An AR15 combo tool
  6. Utility Knife for packages
  7. A mat for building on.
  8. A tray for parts. Old pill containers can be awesome for this.

With all of those things ready, let’s build a gun! 

Magazine and Bolt Catch

The magazine release button and spring both drop into the relevant hole without much effort, so it’s a good place to start: the serrated side of the button faces out. Hold the button, and turn the receiver over and then thread the magazine catch on. Keep threading until the catch sits flush when you release the button. If you have a magazine around, you can test it to ensure a good fit before you put any more parts in.The bolt catch goes on with a spring that fed in from above the magazine catch. From there, get the bolt release oriented correctly before driving the roll pin through. For doing this, some needle nose pliers to press in the pink help a great deal. To avoid marring the receiver, wrapping the pliers in an old cloth is a good idea.

Trigger

Some precision triggers drop in with just two roll pins. If that’s your setup, we’d recommend finding a more specific guide for your exact model. Also, some lower receivers do not come with a trigger guard and require an aftermarket one that similarly goes on with pins. Again, there is some variance and thus you’ll need to do some more digging.

Assuming you’re working with a standard, mil-spec trigger, start with threading the springs on the trigger and sear as you can see in many online guides. Then, lay the fat side of the disconnect spring down onto the trigger and loop the sear into the correct position. Following, drop the assembly into the receiver until the holes line up. Driving the trigger pin in is typically easy until about the last tenth: from there, a punch and a hammer will help get it the rest of the way though. Now, you can and should test your trigger and reset to make everything work correctly once the gun is assembled.

Grip, Safety, and Buffer

Now is a good time to assemble the safety, which involves putting the selector lever in, followed by the detent spring in the bottom of the receiver. This spring is held in by the pistol grip, which typically attaches by either a screw or a hex nut. Either way, this is a point where being able to clamp the receiver upside down comes in very hand as this is a three-handed job.

To get the buffer working, this first step is to install the rear takedown pin: the pin goes in the hole, the detent spring goes in through the back of the receiver after the castle nut. From that point, the buffer retainer goes in a hole that is in the threading inside of the rear of the receiver. Now, place the end plate over the threading on the buffer tube. A note on the endplate: one side has a little channel for the takedown spring, make sure that end faces the receiver. Hold the end plate with one hand and screw on the buffer assembly. With the other hand, keep the endplate facing the correct spot on the back of the receiver. Stop screwing just before the buffer tube reaches the detent inside of the receiver extension.

Before you tighten the castle nut, make sure that the nd plate is positioned correctly: then, using an AR wrench, get the castle nut hand tight. This is all of the tension it needs to work well. Some people will stake the castle nut at this point with a punch. This is a generally good idea, but function check the rifle first.

A few final notes for your first AR build. First and foremost, expect to lose some springs and have to order them again. Some companies sell small spring kits just for this purpose and getting one is a great use of a few dollars.

Second, as you’re doing this, it’s a good idea to have a detailed video open. A well-written guide is a lot of help, but watching someone do it in real-time is another excellent tool.

As far as workspace goes, I always try to have a space all to myself that’s quiet and well-lit. That makes putting together parts with small, fiddly springs a lot easier for most people.

With all of that said, it is certainly possible to build an AR lower even without having handled one before, so go for it!

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