AR15 Lower receiver assembly guide

[Wallace's]

Just figured I would put this out there since I have seen a couple threads pop up with lower receiver assembly issue from a few members. I made this earlier in the year for another site, but figured I would share it here too.


This is not the "must do" method, just how I assemble them with the tools I find useful. There are other methods, and many assemble with alot less tools than I do, but I have found the below selection makes assembly a breeze and least likely to damage your receiver.

I have this available via .pdf as well for those that may want to save it on your desktop. (apologies in advance for typos...)

Below is the layout of the tools I use to assemble lower.

Be sure to lay your lower build kit out and inventory everything to ensure you received the proper parts and quantities. I find it easier to group the parts together how they install. Missing or wrong components become very obvious this way.

Inspect your lower to make sure all of the holes are present and extra finish buildup is not found in the holes. Also, be sure to pre-install the grip screw to ensure the threads are clear before you install the grip later on. If you see heavy coatings in holes that may prevent installation of some components, use the file handle or a pin vice to clean the holes up with the appropriate sized drill bits. Do not use a cordless drill for this, turning by hand will be enough to clear out 99% of obstructions. You can chase the grip screw threads with a ¼-28 tap if the threads have excessive buildup. If you notice missing holes or very out of spec sizes/spacing, contact the manufacturer/supplier to replace your unit with a new one contingent on their warranty coverage.

Below I am clearing out a small amount of excess finish buildup in the trigger roll pin holes.

Install the set screw first using a 1/16 hex key. If your set screw does not have Loctite applied from the factory, use one small drop of Loctite 242 (blue) on the screw before install. Screw in until you feel it bottom out. The screw will be a little below flush. Do not over tighten.

 

[Wallace's]

Next, insert the trigger guard roll pin into your roll pin starter punch

Be sure you have the lower roll pin tabs supported. I use the Wheeler bench block to do this. Note how the bottom tab is supported with the block (red arrow). Once supported, use the starter punch to drive in the pin until the punch is just above the receiver.

Finish driving in the roll pin with a 1/8 roll pin punch until it is just below flush.

Next, install the bolt catch spring and plunger/buffer into the recess in the receiver.

I prefer to lightly grease the sides of the bolt catch and contact point where the buffer contacts the bolt catch itself.

I find using a small, right angle 5/16 hex key makes a great slave pin to keep the bolt catch aligned with the holes for the roll pin installation.

 

[Wallace's]

Next, tape the receiver with painters tape to cover the area between the bolt catch and receiver extension. I find wrapping the tape over the back of the receiver helps further protect it from hammer marks in case you miss the punch.

Using a starter punch, start to drive in the bolt catch roll pin. This will cause the hex key fall out once the pin has been pushed in enough

Next, use a 3/32 roll pin punch to finish driving in the roll pin. Press in until the pin is equally recessed on both sides. Remove tape.

The correctly installed bolt catch roll pin will be just below the surface as shown below on both sides. I depress the bolt catch several times to ensure it moves freely in the lower without binding.

Next, the magazine catch will be installed. Install the spring into the recess inside the receiver.

 

[Wallace's]

Next, install the magazine catch into the left side of the receiver until flush.

I use my fingers to align the mag release bottom with the threaded portion of the magazine release. I also make the first 2 revolutions by hand to ensure I do not cross thread the two and makes threading it up a little easier.

Once the first two revolutions are made, I use a wooden dowel to depress the mag release button fully into the receiver. This allows the catch to clear the receiver/bolt catch while turning in. Rotate the catch until the threaded post is just below flush in the button.

Note: The magazine catch threaded post is just under flush on the button.

Insert an unloaded magazine into the lower and function test the magazine catch. Make sure your magazine is held into the receiver once inserted. Also, that it drops free from the receiver once the release is depressed.

The front pivot pin is the next component to be installed. It differs from the rear takedown pin from its extra length and flat ground head side surface.

 

[Wallace's]

The below example shows an install using a ¼” clevis pin. The pivot pin can be installed without this (I only recommend installation without a pivot pin installation tool for more advanced builders). Drop the detent spring into the hole of the clevis pin and down into the receiver detent/spring hole.

Follow the spring install with the detent

Use a small punch to depress the detent and spring below the clevis pin opening then rotate the punch 90 degrees. Remove the punch.

I like to prep the pivot pin by lightly greasing the detent groove with Tertra Gun Grease. Just a small dot is all that is needed.

Take the pivot pin, and align it with the clevis pin. Firmly grasp the outer sides of the pivot pin and support the botton with another finger and push the take down pin into the receiver holes. I apply light pressure to the clevis pin with my palm (blue arrow) while applying more pressure to the pivot pin (red arrow) to ensure a gap does not appear in between the two causing the detent to fly out.

 

[Wallace's]

Rotate the pivot pin until the detent pushes into the groove of the pin. You can lightly oil the outside of the pin with CLP to make movement slightly easier. Slide it back and forth a couple of times to test for proper function.

The FCG will now get assembled/prepped for install. Locate the trigger and disconnect spring (typically green and always tapered). Insert the disconnect spring into the trigger (red arrow), widest part down (blue arrow).

Ensure the spring is fully seated into the trigger with the widest part of the spring fully in the bottom of the spring recess.

Install the trigger spring so that the the loop portion is under the sear mating surface (blue arrow) and legs of the spring are facing towards where the magazine well would be (red arrows).

Locate the hammer, and install its spring. The legs of the spring should be in the forward position (blue arrow) with the loop portion being closest to the front (barrel side) of the receiver (red arrow).

Below shows the incorrect installation of the hammer spring (loop and legs towards the rear). Installation of this spring backwards can cause light primer strikes and the failure of proper rifle operation.

 

[Wallace's]

Next, drop the trigger and disconnector into the lower receiver, guiding the trigger through the receiver opening in the bottom.

I use the Geissele Trigger Fitting Pin as a slave pin to align the trigger and disconnect with the holes in the receiver. This can also be done with a 5/32 punch. The Geissele tool is tapered on the end to make aligning the disconnect mush easier/quicker.

Once aligned, push the trigger pin into place.

Once the trigger pin is installed, it should be flush on both sides of the receiver.

Installation the hammer requires that you seat the spring legs over the trigger pin (red arrow) and pushing down and forward to align the holes in the receiver (blue arrows). This ensures that the trigger pin does not fall out during use and that the hammer has enough force to reliably operate the firing pin.

I use the Gessele trigger fitting pin to align the hammer with the holes in the receiver. I push the hammer pin in by hand until it gets to the “J” spring in the hammer. I then lightly tap the pin with the pin to get it past the “J” spring. Once its past that point, I push the pin into the receiver until I feel the “J” spring seat in the middle relief of the hammer pin.

 

[Wallace's]

I then function test the FCG to ensure the disconnect and sear are properly functioning. DO NOT dry fire an AR lower without a complete upper receiver installed. Dry firing and AR without a hammer block or using your finger to slowly release the hammer can break your bolt catch and or damage your lower receiver.

With the hammer in the cocked position, install the safety selector until it is fully inserted into the receiver. Again, be sure not to dry fire the lower at this point. Release the hammer while guiding the hammer forward by hand.

Now, prep the castle nut for installation by first degreasing the threads, then applying Aersohell 33MS to the threads with a brush evenly. I have found a plumbers flux brush works best for this. Applying grease to the castle nut threads will spread the grease out over the receiver extension threads and retain some in the castle nut itself for install.

Spin the castle nut onto the receiver extension with the wrench slots facing the rear of the extension (red arrows) and the staking indentions towards the front (blue arrows). Spin the castle nut to the rearward most portion of the tube until it stops, and slide the end plate on with the receiver extension with the alignment tab facing outward towards the receiver (green arrow). This ensures you can rotate the receiver extension into the receiver without interference of the rear take down pin detent spring. Set the assembly aside.

Next, we will prep the rear take down pin for install. Again, I prefer to lightly grease the detent slot of the takedown pin with a dot of Tetra gun grease before install just like the front pivot pin.

Slide the rear take down pin into the receiver with the detent groove facing the detent/spring hole in the rear of the receiver.

 

[Wallace's]

Insert the take down detent into the rear of the receiver and into the hole, followed up by the spring.

Install the buffer retainer and spring into the receiver hole (you can do this by hand, I used pliers to make the photo easier to see) and start the receiver extension into the receiver threads.

Using a flat head screw driver or your thumb, depress the buffer retainer enough so that its shoulder sits below the receiver extension threads in the receiver.

Turn the receiver extension assembly in until the bottom lip of the tube covers the buffer retainer’s shoulder fully.Depress the retainer to ensure it moves freely one the receiver extension is fully installed.

Align the end plate with the indention in the receiver and slide forward being sure that the rear take down pin detent spring goes straight into the receiver.

 

[Wallace's]

While holding the end plate up against the receiver (red arrow), tighten the castle nut against the end plate by hand until snug.

The TM calls for 40+/- 2 foot pounds. I do not stake the castle nut at this point in case you end up needing to remove the receiver extension assembly for miscellaneous reasons.

Flip the receiver over on the vice block to install the safety selector detent. Pointed end towards safety, and flat portion towards the grip. I lightly grease the end of the detent before install.

I apply a larger amount of grease to the safety detent spring to help hold it into the grip.

Flipping the grip upside down, I install it onto the receiver being sure to align the spring into the safety detent hole.