We Have An AR At Home, Part 4: Assembling the Lower Receiver

With tools and parts in hand, it’s now time to start putting things together. For those readers who are interested in a deeper dive into everything AR, I highly recommend the SOTAR (School of the American Rifle) YouTube channel.

I’m going to start with assembling the lower receiver, as it's the most involved portion of the process. My first step before doing this is to make sure the Lower Parts Kit (LPK) came with all the parts; I have unfortunately received LPKs with bits missing.

If any small parts are missing, or they go missing during assembly, several manufacturers offer sets of the most commonly lost parts. One such is the appropriately named Ooops Kit from Aero Precision. They also offer their slightly more comprehensive Field Repair Kit.

To help students in my classes with parts identification and organization, I put together the following diagram.

The hammer and trigger pins are identical and interchangeable, as are the takedown and pivot pin detents and springs. The selector switch detent and spring are different and should not be confused with the others.

Once the parts are all present and accounted for, assembly can begin.

Magazine Catch

One of the easiest steps to start with is installing a standard magazine catch. This involves three parts: the magazine catch, button, and spring. Use blue painter's tape to protect the anodizing from scrapes.

1. From the right side of the lower, insert the spring and button.
2. Using a punch, hold the button all the way in the receiver and insert the catch from the left side.
3. Rotate the catch clockwise until the threaded portion starts to bear against the punch or the catch bar gets very close to the receiver.
4. When properly installed, the end of the threaded part should be just below flush with the face of the button.

Now that the magazine catch is installed, most of the following steps can be completed with the Lower Receiver Vice Block in place. Place the vice block in the magazine well and secure it in a vice with the front of the receiver facing up.

Forward Pivot Pin

Installing the pivot pin has been known to cause considerable profanity, but the method I use has had good results. Placing a large clear plastic bag over the end of the receiver during installation can reduce the chance of lost parts.

The pivot pin spring and detent can launch at high velocity!
 Please wear safety glasses when completing this step!

1. From the left side of the receiver, slide the Clevis Pin through the pivot pin ears.
2. Line one of the clevis pin holes up with the pivot pin spring channel and drop in the spring.
3. Place the detent on top of the spring.
4. Using a punch or hex key, drive the detent into the channel until it is completely past the clevis pin.
5. Using the punch as a handle, rotate the clevis pin so it holds the detent in place.
6. Line up the pivot pin with the end of the clevis pin. Make sure the flat side of the pivot pin end cap is lined up with the spring channel.
7. Firmly place a finger in the junction between the two pins opposite the force of the spring.
8. Carefully use the pivot pin to push the clevis pin past the spring channel. 
9. The detent should snap into the slot in the pivot pin. 
10. Make sure the pivot pin slides freely back and forth in the receiver, but is held in place at both ends of travel. 

Bolt Hold Open

Installation of this assembly can be a bit stressful, but if taken slowly and with proper care, it will turn out well. Use blue painter's tape to protect the anodizing from scrapes.

1. Using a proper size roll pin punch, start the roll pin in the forward bolt hold open lug on the receiver and drive it in just enough to hold.
2. Put the bolt catch buffer and spring together, and insert them into the relief cut in the receiver.
3. Place the bolt hold open over the buffer with the grooved pad towards the top of the receiver. 
4. Use another punch, or a short section of drill rod, to keep the bolt hold open lined up.
5. Using a proper size roll pin punch, gently tap the roll pin the rest of the way through until it is flush on both sides.

For the next steps, the vice block should be repositioned in the vice so the lower has its top facing up.

Fire Control Group

Trigger Assembly

1. Place the smaller of the two-legged coil springs on the sides of the trigger, with the loop under the front of the trigger bar.
2. If necessary, install the disconnector spring in the rear of the trigger bar. The wider end of the spring goes in first.
3. Position the disconnector in the trigger bar with the hook facing forward and the holes aligned. 
4. Use a short section of drill rod, or even part of a toothpick, to keep the disconnector and trigger lined up.
5. Place the trigger assembly in the receiver, making sure the legs of the trigger spring are flexed forward.
6. Drive the trigger pin through until it is flush on both sides.
7. Confirm the trigger returns forward when pulled.

Hammer Assembly

1. Place the larger of the two-legged coil springs on the sides of the hammer, with the loop to the rear and the coil running over the top.
2. Place the hammer into the receiver, making sure the legs of the spring are flexed backwards and rest on top of the trigger pin. 
3. Drive the hammer pin through until it is flush on both sides. 
4. Carefully cock the hammer and make sure it engages with the trigger bar.
5. While controlling the hammer with a finger, pull the trigger to release the hammer.

Do not let the hammer drop against the receiver with full force!
It can crack the receiver!

Buffer Tube & Rear Takedown Pin

Buffer Tube

This is the process for installing an adjustable stock receiver extension, also known as a buffer tube. 

1. Thread the castle nut all the way onto the receiver extension, notched end first.
2. Followed this with the receiver plate, with the raised portion towards the open end of the extension. 
3. Start threading the receiver extension into the lower receiver.
4. Stop before the extension reaches the buffer retainer relief inside the receiver.
5. Insert the spring and buffer retainer, then screw the extension just enough to catch the edge of the retainer. 
6. Do not rotate the extension in completely or adjust the castle nut yet. 

Rear Takedown Pin

1. Insert the rear takedown pin from the right side of the receiver, slotted side to the rear. 
2. From the back of the receiver, insert the other detent and spring.
3. Make sure the receiver plate can clear the detent spring, then rotate the extension into place.
4. Carefully push the receiver plate forward, compressing the spring into the receiver.
5. While holding the receiver plate, screw the castle nut firmly into place.
6. Make sure the pivot pin slides freely back and forth in the receiver, but is held in place at both ends of travel. 

For the remaining steps, the receiver can be removed from the Lower Receiver Vice Block. 

Butt Stock

This step is very easy.

1. Position the butt stock on the receiver until it stops.
2. Pull the lock lever out until the internal pin clears.
3. Slide the butt stock the rest of the way onto the buffer tube and release the lock lever.
4. Check to make sure the butt stock both adjusts and locks smoothly. 

Trigger Guard

The AR trigger guard is designed to fold down against the pistol grip so that it may be operated while using mittens or heavy gloves. This means the roll pin needs to go through the rear of the trigger guard.

Never use any force on the trigger guard ears without the trigger guard in place. 

Doing so can break the receiver.

1. Place the receiver leftside-up on a firm surface.
2. Position the trigger guard in the receiver with the pass through hole at the rear and the front spring pin locked into the front of the trigger guard opening.
3. Using a properly sized roll pin punch, start the pin into the trigger guard.
4. Confirm everything is lined up, then gently tap the roll pin the rest of the way through until it is flush on both sides.

Safety and Pistol Grip

1. With the receiver still leftside-up, maneuver the safety lever over the trigger bar until it sits flush.
2. Turn the receiver upside down and place the detent point-first into the hole below the safety lever.
3. Make sure the safety lever stays in either the safe or fire position.
4. Place the safety spring into the hole in the top of the pistol grip.
5. Guide the spring against the detent while positioning the grip onto the receiver.
6. Install the lock washer on the grip screw, then place the screw on the proper driver bit. 
7. While holding the grip in place, lower the receiver onto the grip screw and fasten it snugly, but not too tight. 

At this point, the lower receiver is complete and can be set aside while we put together the rest of our rifle.

Stay tuned for more adventures in AR assembly!