Building an AR-15 Part 2: Lower Receiver Assembly

Yesterday, in part one of our Building an AR-15 series, we covered the types of AR-15s that can be built and some of the tools needed. Once you have decided on what kind of AR-15 you want to build, you are ready to order parts. Local gun stores often carry a lot of the parts needed, or they can be ordered online.

The lower receiver is the part of the gun that makes it functional, according to the ATF. Because of this, the lower receiver must be purchased from an FFL dealer (gun store). The lower receiver can be ordered online if you can’t find one locally that you like. It will have to be shipped to a local gun store where it will be treated the same as a firearm. Everything else can be shipped directly to your home.

Lower parts for AR-build
The only parts needed for the lower receiver assembly are the buffer tube and spring set, lower parts kit, and the lower receiver itself.

Ordering Parts

I like to order the lower and upper receivers together from the same company to make sure the colors match. Companies use the same name for colors, but they are often different. I ordered my stripped lower and upper from Aero Precision. The lower I selected is the M4E1 Enhanced Forged Lower in FDE (flat dark earth).

Next, purchase the lower parts kit. This kit includes everything needed to assemble the lower receiver. Other required parts you will need are the buffer tube, spring, and adjustable stock. Part 4 covers the installation of the buffer tube and spring.

Here is a list of what you’ll need:

  • Stripped lower receiver
  • Lower parts kit
  • Buffer tube and spring set
  • Adjustable stock that fits mil-spec buffer tube

Installing the Mag-Release Assembly

If you have a vice and vice block as mentioned in Part 1, the lower receiver should be placed on the vice block. If you do not have a vice, you will need someone to hold the lower receiver up while you work. I purchased the Anderson Manufacturing lower parts kit for my build. From the lower parts kit, you will need both pieces of the mag release and the largest spring out of the parts kit. Most kits mark the pieces for you to help identify them easier.

mag release parts
The mag release assembly includes (left to right) the spring, mag release button, and mag release/catch

When installing the mag-release assembly, you first insert the mag release (pictured above on the right). It will fit into the groove on the lower receiver and sit flush. The spring will then go into the hole on the other side and slide over the threaded part of the mag release. Next, the mag button will thread onto the mag release, compressing the spring as it does.

You will only be able to turn the mag button a couple of times before you run out of room to turn it. Once it is on the threads, push the button in all the way with the back end of a punch and turn the mag release on the other side. Turn it about seven times or until it sits flush. You can test it by pulling it on and off the vice block. If you must push in the mag release to get it off, it is working correctly.

Trigger Guard

Most mil-spec lower receivers will need to have a trigger guard installed. The lower receiver I purchased has a fixed trigger guard, so it is not needed. If yours does need a trigger guard, however, you simply attach the end with the spring-loaded pin first and then line the holes up for the roll pin to be inserted on the other end. This is the largest roll-pin on the AR-15 and takes a small hammer and 1/8-inch roll-pin punch to get it in. Once the holes are lined up, lightly tap the roll pin until it is flush on both sides.

Trigger guard installation - building an ar-15
The trigger guard is easy to install with the use of a small hammer and a 1/8 roll pin punch. You may have to tap a little harder on this roll pin than anything else on the lower assembly.

Installing the Bolt Catch Assembly

The bolt catch assembly includes the bolt release, roll pin, spring, and plunger. To install the assembly, first insert the spring into the hole above the mag release. The plunger will then go in the same hole, compressing the spring as you push it in. The larger rounded side faces outward. The bolt release then fits in the groove and pushes the plunger and spring further in place. Be sure to keep pressure on the bolt release until the roll pin is in place or the spring will come flying out. Once the bolt release is lined up with the holes, the roll-pin can be inserted and then tapped into place.

Bolt catch assembly
The bolt catch assembly consists of the bolt catch (top), spring, roll pin, and plunger (left to right).

The best way to do this is to use a roll pin holder, which looks like a punch but has a hole at the end that holds the roll pin while you line it up and tap it in part way. Once the pin has been started, you can use a roll pin punch to finish tapping it in until it is flush. Take this part slow or you can scratch up the side of the receiver with the punch and hammer.

Installing the roll pin for the bolt catch - building an ar-15
Once the spring, plunger, and mag release/catch are in place, the roll pin can be slowly tapped in. I use the Real Avid roll pin holder to get the pin started. Once the roll pin is lined up with the bolt release, tap it in the rest of the way until it is flush.

Takedown Pins

There are two takedown pins on the lower receiver. One in the front and one in the back. These are what connect the upper receiver to the lower. The rear takedown pin must be installed along with the buffer tube, so we will only install the front take down-pin for now. The front takedown pin is the longer of the two. It will also have a small spring and detent pin. These are not hard to pick out of the lower parts group because both detents and springs are the same. The small detents have the shape of a bullet and are brass.

Takedown pin installation in building an ar-15
The Real Avid Pivot Pin Tool makes installing the takedown pin easy. It comes with step-by-step instructions. They also have videos on YouTube explaining how to use the tool.

Installing the front takedown pin is the most difficult. The Real Avid Pivot Pin Tool makes this much easier. If you do not have one, you can also use a 1/4-inch clevis pin from the hardware store. You start by inserting the spring into the small hole next to the double eyelet (where the takedown pin goes) on the front of the receiver. Then insert the small detent pin after the spring. The tricky part next requires the detent to be pushed inside the small hole, allowing the takedown pin to pass through the eyelet. The takedown pin has a flat side that goes against the detent. Once the takedown pin is clear of the detent, the pressure from the spring will lock the takedown pin in place when it is closed or open.

Watch for Part 3

We are now ready to tackle the trigger group in the lower receiver. This will be covered in Part Three and the buffer tube and spring in Part Rour.  After that, the fun part of installing the barrel and upper components will begin. This is when you start to see the gun come together. Going through the process of building a rifle can be frustrating, but worth every bit when you have a finished gun that you built with your own two hands.

Sheriff Jason Mosher is a law enforcement generalist instructor as well as a firearms and tactical weapons trainer. Jason graduated from the FBI-LEEDA (Law Enforcement Executive Development Association) and serves as a Sheriff for his day job. When he’s not working, he’s on the range, eating steak, or watching Yellowstone.

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