• Post author:
  • Post last modified:September 18, 2023

It was first designed for the United States Armed Forces as they needed a lightweight rifle that has a higher ammunition capacity. However, ArmaLite didn't manage to attract the intended buyers, so they sold this design to Cold, one of the oldest American firearm manufacturers.

Colt added some modifications to the AR15, and in 1963 they offered it to the American military again as the M16 - this time, it turned out to be a success. Simultaneously, the producer created a semiautomatic version of the AR15 that was destined for the civilian market.

If you already have this rifle and would like to install a lower receiver on it, we have prepared a comprehensive step-by-step guide that will help you to achieve this in no time. Good luck!

Installing the AR15 Lower Receiver - Step-by-Step Guide

This is a how-to assemble AR15 lower guide from start to finish. Before starting, you may need these tools to help you successfully put together everything.

Pliers on a black table

Obligatory Tools:

  • Standard Punch Set
  • Universal Bench Block
  • SAE Allen Wrenches
  • Nylon or Brass Hammer
  • Flathead Screwdriver
  • AR Armorer's Wrench
  • Needle Nose Pliers
  • Roll Pin Punches
  • Vise With Soft Jaws
  • AR Lower Receiver Vise Block

Optional Tools:

  • Razor Blade
  • Painter's Tape
cmc lower parts kit with trigger

Stage 1 - The Trigger Guard Assembly

The first element we are recommending installing on your AR15 lower receiver is the trigger guard. In this stage, you will need your trigger guard along with the trigger guard roll pins and the trigger guard screw.

Look at the bottom of your lower receiver and find the trigger guard hole to place the trigger guard there.

Use the specified Allen wrench (1/16'' would be best) to mount the trigger guard screw in the hole at the front of the trigger guard slot, usually placed near the magazine well.

Then, place the other end of the trigger guard in the rear opening of the hole.

With the help of a nylon or brass hammer and a roll pin punch, finish mounting the trigger guard by driving the trigger guard roll pin through the hole in the receiver.

Stage 2 - Magazine Catch and Button Installation

For this stage, prepare your magazine catch, magazine catch spring, and magazine catch button.

Start with putting the magazine catch in the long narrow slot on the left side of the receiver. You will see it just below the recess for the bolt catch spring and buffer.

While firmly holding the magazine catch in place, slide the magazine catch spring over the post for the magazine catch, going through the magazine catch button hole.

Then, put the magazine catch button into its hole and push it, generating the magazine catch spring tension and expanding the magazine catch out from the other side.

When it's finally where it should be, turn the receiver over and line up the post with the hole inside the magazine catch button.

Make sure the magazine catch's arm is pressed away from the side of the lower receiver. As a result, you will be able to rotate the arm of the magazine catch clockwise and screw it in the spot at the back of the magazine catch button.

To facilitate this task, pick a standard pin punch that is a little smaller than the magazine catch button's opening, and push the button into the receiver as hard as you can with it.

Keep on screwing the magazine catch inside up to the moment you notice the tip of its post reaching just below the surface of the magazine catch button's face.

Note that both the magazine catch's arm and the tip of the post should be flush with their respective sides when the assembly is over.

Test the Magazine Function

To make sure everything works well, place an empty magazine into the magazine well on the receiver. When it is already there, pull the magazine gently to check if it seats and if it is retained. Next, push the magazine catch button to check the magazine release.

Stage 3 - The Pivot Pin Installation

Before you grab one of the pivot pins, secure the lower receiver in your vise. In order to do it, place the front end of the receiver facing up.

Put the pivot pin detent spring in the hole you will see at the front face of the receiver.

Then, use your needle nose pliers and grip the pivot pin detent in order to press it against the spring. Then, drive it along with the spring down into the hole of the receiver.

Take a thin razor blade-style knife and press the side of the blade onto the top of the detent to grab it.

When both the spring and the detent are held safely in place, you need to slide the pivot pin over the side of the razor blade to place it in the hole of the receiver. During that process, ensure to hold the groove in the pivot pin down toward the head of the detent to get the pin and keep it in place when the assembly is over.

Gently remove the razor blade from underneath the pivot pin and seat the pin firmly in its slot in the receiver. Make sure that the flat side protrudes a bit on the other side of the receiver. The rounded pivot pin head is supposed to fit into the recess on the receiver, while the flattened edge of the pivot pin head is turned toward the rear of the gun.

To make sure the detent holds the pin in place, try to pull it out. Next, reset the mechanism and check the function.

Stage 4 - Bolt Catch Assembly

It is recommended to cover the area around the bolt catch with painter's masking tape before you install the bolt catch. Although it's not obligatory, it will allow you to avoid marring the finish on the receiver.

When you already protect the area, start by partially driving in the bolt catch roll pin. You can do it using a hammer and a roll pin punch. For now, you just need to get the pin started enough to be retained in the receiver, checking if the roll pin does not extend into the slot housing the bolt catch.

Then, put the bolt catch spring inside the hole below the roll pin on the receiver.

This is also where you should place the bolt catch buffer. Check if the thin post on the buffer goes down into the hole with the rounded end on the buffer turned outward.

Next, place the bolt catch inside the recess with the ribbed paddle turning to the left side of the receiver.

Push the bolt catch into place up to a point when the hole in the side of the bolt catch lines up with the hole in the lower receiver where the bolt catch roll pin was driven before.

Put a pin punch inside the hole in the receiver on the opposite side of the roll pin to hold the bolt catch.

With your nylon or brass hammer and a roll pin punch, drive the roll pin through the hole in the bolt catch to end the procedure.

In order to test the function, push the ribbed paddle. When you do it, the paddle should spring back. If everything works all right, take the painter's tape off and proceed to the next step.

Stage 5 - Trigger Group Installation

The first step of mounting the trigger group is fitting the hammer and trigger springs. When it comes to the hammer spring, you should assemble it with the coils around the posts on the side of the hammer.

The squared loop in the spring needs to be wrapped around the hammer on the hook side and the legs of the spring need to extend down past the hammer's bottom.

You should install the trigger spring with the coils around the posts on the sides of the trigger. Make sure that the legs of the spring extend down away from the trigger, the feet on the end of the spring point up, and the squared loop on the trigger spring fits under the front protrusion.

Disconnector and Disconnector Spring Assembly

Keep in mind that the disconnector spring needs to be mounted in the recess at the rear of the trigger.

Note that one end of the disconnector spring is a bit wider than the other. Check if the wide end is turned down to lock the disconnector spring in place.

When the spring is where it should be, the disconnector is assembled on the top of the trigger.

The square notch in the underside of the disconnector should rest at the top of the disconnector spring.

As a result, the trigger assembly should be put into the lower receiver with the interior of the trigger bow turned to the front of the lower receiver.

Finally, line up the trigger, the hole in the disconnector, and the trigger pin hole in the receiver by pressing down the trigger assembly.

Trigger Pin Installation

Mount the trigger pin from the outside, on the left side of the receiver. Take a look at the trigger and hammer pins - you should see that both of them have ringed notches. Install them with the longest part of the pin going inside the receiver first.

So as to keep the trigger and the disconnector lined up more effectively, you can insert a punch from the right side of the receiver. Use your nylon/brass hammer to tap the pin into place and flush on both sides of the receiver.

Once you mount the trigger, it should spring back when you press it from within the trigger guard.

Hammer Assembly

You need to install the hammer with its hook side turned to the rear of the receiver, and its flat face turned forward. When you put the hammer inside the receiver, remember to let the legs of the hammer spring rest on the coils of the trigger spring so that when you press the hammer into place, the spring tension can drive the hammer into the forward position.

Next, push the hammer down up to a moment, the hole in the side of the hammer can be lined up with the hammer pin hole in the side of the receiver. Now, proceed to assemble the hammer pin in the same way as you did with the trigger pin.

When you install both the hammer and the trigger, test the function and pull the top of the hammer down to the rear in order to lock it into the ready-to-fire position. To prevent the hammer from striking inside of the receiver with its full force, brace its face first. Make sure the assembly went well by testing the function multiple times.

Stage 6 - The Pistol Grip & Safety Selector Switch Installation

Place the selector spring inside the hold at the top of the pistol grip. Set the pistol grip and spring aside.

Then, grab your lower receiver and turn it upside down.

Place the selector detent inside the hole in the underside of the lower receiver on the right side of the pistol grip part. If you mount it correctly, the detent's pointed end will be turned toward the opening for the selector switch.

Next, put the selector switch inside the selector switch hole that you can find on the left side of your lower receiver. Make sure the selector's tip is pointing to the firing position.

Keep the selector in place and then put the pistol grip in place, checking if the selector spring lines up with the selector detent hole.

Pay attention not to kink/bend the selector spring because it can result in sloppy tension on the selector detent.

The final step of this stage involves inverting the lower receiver assembly and mounting the pistol grip screw (provided that you have the pistol grip lock washer in place around the screw post).

When you already have the pistol grip in the right place, test for the function on the selector switch. You should feel a click from the selector detent when you switch from safe to fire positions.

Stage 7 - Takedown Pin Assembly

To mount the takedown pin, put the pin into the takedown pin slot you can find at the back of your receiver. Make sure that the channel notch in the pin is positioned toward the rear of the receiver. Then, insert the takedown pin detent and the takedown pin spring in the rear face of the receiver.

The AR15 rifle

Stage 8 - Buffer Tube Assembly

If the end plate and the castle nut are pre-threaded onto the buffer tube, start screwing the buffer tube into the rear of the receiver. The castle nut needs to be set in such a way so that the larger notches on the edge of the castle nut are turned to the rear of the buffer tube and the smaller notches that are supposed to be used for staking the castle nut are turned to the front of the receiver.

Screw in the buffer tube up to a point you notice that the end plate catches and compresses the takedown pin spring. Thread the castle nut down so that the end plate can be safely in place.

When the front lip of the tube is already less than flush with the hole in the top of the buffer area of the receiver, stop threading in the buffer tube.

Now it is time to enter the buffer retainer spring and then the buffer retainer into the hole at the top of the receiver.

Use a small flat-bladed screwdriver or simply your fingernail to push down the buffer retainer into the hole. Next, continue threading in the buffer tube up to a point when the lip of the buffer tube covers the outer rear edge of the buffer retainer to keep it in place.

If you see that the holes in the underside of the buffer tube which interact with the stock are in line with the rest of the lower receiver, it means your installation went well!

At the very end of the buffer tube assembly, put the receiver inside the vise and tighten the castle nut to 40 ft/lbs of torque with the help of an AR armorer’s tool.

Note: Be sure to know the difference between mil-spec and commercial buffer tubes.

Stage 9 - Staking the Castle Nut

For staking the castle nut, you need either a standard center punch or a spring-loaded center punch, and a hammer.

First, put the tip of the punch on the lower receiver end plate. It should be a point at the top of the end plate that is in line with a small staking notch you can see on the castle nut.

Hold the punch straight up and down and strike it to make a dimple in the end plate. When you already have a dimple, turn the punch to around 45 degrees with the top of the punch angled toward the front of the receiver.

At that moment, you need to strike the end plate to press the metal into the staking notch in the castle nut so as to avoid the castle nut backing out.

Possible Alternative - Blue Loctite

If you want to avoid staking, we can suggest using a blue Loctite on the inside of the castle nut as an alternative.

Staking is usually perceived as the standard method to keep the castle nut in place and protect it. Nevertheless, some people may not be willing to destroy their end plate, or they prefer to switch out their buffer tubes with specialty ones to be able to use specialty buttocks or pistol braces. In such cases, you may benefit from blue Loctite instead of staking your castle nut.

However, take into consideration that such a method is riskier than proper staking, so you have to think if the tradeoff between security and versatility is really worth it.

When you finish installing the buffer tube, you can assemble an AR buttstock in accordance with the producer's guidelines.

The Buffer and Buffer Spring Insertion

Proceed to the buffer tube - you need to enter the buffer and the buffer spring with the wide flat metal face of the butter turned to the front of the receiver.

When you push the buffer into the position behind the post on the buffer retainer, you should see there are both round and flat parts around the outer edge of the buffer head.

Turn one of the flat parts down to pass over the buffer retainer post. When the buffer head has completely cleared the post on the buffer retainer, turn the head of the buffer in such a way its rounded section is oriented down and will stay retained by the buffer retainer.

Then, after inserting the buffer and the buffer spring, proceed to the last step of the AR15 lower receiver assembly.

Stage 10 - Final Touches and Safety Checks

So, you basically have your lower receiver fully installed! Don't hesitate and pull out the pivot and takedown pins, and add the completely assembled upper receiver to the lower. Push the takedown and pivot pins in order to lock the upper and lower receivers together.

Then, you have your firearm complete! However, do never forget to perform all the necessary safety checks before you start using it. You should make sure the safety selector allows the trigger to drop the hammer in the fire position and will keep that from happening when in the safe position.

If you have conducted all the safety checks, you can take your rifle to the range and perform a proper field test. Have a great time!