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How to Build an AR 15 Lower Receiver: Easy 10-Step Guide

How to Build an AR 15 Lower Receiver

The AR 15 is known for its high degree of modularity, which might give beginners a difficult experience in assembling the AR 15 lower receiver.

However, there’s no reason to fear because we will show you how to build an AR 15 lower receiver in the simplest way possible.

A Complete Step-by-Step Process

Step 1: Assemble the Required Tools

Step 1

The basic tools needed to build an AR 15 lower receiver are:

  • Brass Punch and Punch set
  • Hammer (preferably small brass type)
  • Hex Wrench (preferably t-handle type) or Flat Head Screw Driver depending on the type of Pistol Grip Screw
  • Castle Nut wrench for collapsible stocks
  • Multi-tools
  • Needle nose pliers
  • AR 15 Lower Receiver Fixture
  • Optional Tools: Small Magnetic Parts Dishes

Step 2: Front Pivot Pin Assembly

Step 2

There are many ways you can start with the assembly of your AR 15 lower receiver. However, I find it easier in this guide to start with the hardest part of the build, which is the front pivot pin assembly.

To distinguish the front pivot pin from the rear takedown pin, just remember that the front pivot pin is longer than the rear takedown pin.

Takedown pins are a little bit hard to distinguish, but their corresponding springs are just identical, and you can use both springs for either of the pins.

Installing the Spring and Detent

  • Start by dropping the pivot detent spring into the small hole in the upper right of the AR 15 lower receiver.
  • Take the pivot pin detent and your punch (NOTE: Both have approximately the SAME DIAMETER.)
  • Press the pivot pin detent into the hole with your thumb on top of the spring. (NOTE: Do this step cautiously or else you might lose your detent.
  • Press the tool through the left part until the detent sticks to your punch.

You need to do these steps cautiously because your detent might fly away to some obscure area if you forcefully push it while the detent still hasn’t stuck to the tool.

This would then be difficult to find as it is a tiny component of your AR 15 rifle.

Laying Down the AR 15 Lower Receiver

This will give you an easier time finding the pin once it shoots out.

Carefully push against the protruding punch using your front pivot pin until the detent connects to the pivot pin. Detent should be fully secure to the pivot pin.

Your detent should make a clicking sound and penetrate the holes and bottom of the pin.

If these happen, you would have successfully attached the front pivot pin to your AR 15 rifle.

Step 3: Magazine Catch Assembly

Step 3

Next, we go on and assemble the magazine catch assembly guide. Drop the mag catch on the left part of the AR 15 lower receiver. The magazine catch should fit snugly into the space.

When you take a look at the right part, you should see that the screw should be sticking out through the other side.

Drop the magazine catch spring over the screw and then secure the magazine release button onto the screw by turning it sufficiently to make it hold on.

Press down the button to turn the AR 15 lower receiver over. Be sure to grab hold of the magazine catch on the other side of the AR 15 lower receiver to turn it around. 

You should turn the magazine catch around 3 TIMES.

Depressing the Magazine Release Button

Use a punch to press down on the magazine release button, press down the button completely into the AR 15 lower receiver body.

Twist the magazine catch until you feel strong resistance from the magazine catch spring.

Test the magazine catch button to ensure that the magazine catch completely leaves the magazine sufficiently.

Now, you have now done the magazine catch assembly.

Step 4: Bolt Catch Assembly

Step 4

For the bolt catch assembly guide, begin by setting the bolt catch spring over the uppermost part of the plunger.

Then, drop the unit (composed of the bolt catch spring and the plunger) into the small hole inside the opening to make way for the catch.

Drop the catch into the opening. Double-check and make sure that the 3 holes line up.

Use a thin punch to push through the holes from the left part to ensure that the openings for the catch align perfectly.

Push down on the bolt catch roll pin into the hole on the right part. This is done to help the roll pin stand at the rightmost area.

Hammering in the Bolt Catch Roll Pin

  • Use a hammer to lightly fix the bolt catch roll pin.
  • Using your brass punch to keep the roll pin in place.
  • Test the bolt catch to make sure the bolt catch spring is under tension.

It is important to note that you can assemble the bolt catch with the roll pin even before the preceding steps if you would like to.

Also, when assembling the bolt catch and bolt release, some small brass hammer is what is really preferred. This is to prevent any scratches from the non-brass hammer you would be using.

Congratulations, you have now finished the bolt catch assembly.

Step 5: Safety Selector Assembly

Step 5

For the safety selector assembly guide, start by pushing the selector through the left side of the AR 15 lower receiver.

Turn the AR 15 lower receiver upside-down, and you will observe a hole on the right side within the vicinity of where the pistol grip mounts.

Drop the safety selector detent into the hole on the right side. Then, drop the selector spring into the corresponding hole onto the upper right part of the pistol grip.

Place a washer on your grip screw and then stick the unit on your hex wrench.

Then, orient the AR 15 lower receiver sideways so that both selector detent and the grip screw are secured in the AR 15 lower receiver or grip.

Otherwise, your selector detent and grip screw might fall out of the AR 15 lower receiver or your grip.

Step 6: Grip Assembly

Step 6

For the grip assembly guide, lightly push the pistol grip onto your AR 15 lower receiver, leaving a small gap of around half a centimeter.

You’ll know when to stop if you experience the following:

  • You feel greater resistance
  • You aren’t able to push lightly.

Do not stick it in completely yet as you still have to check if the selector spring and the grip is pushing up on the detent in the AR 15 lower receiver.

Once you double-check if everything is set, stick the pistol grip completely into your AR 15 lower receiver.

Then, use your t-handle wrench and install the screw through the bottom of the grip.

It is important to note that having a t-handle wrench would make it easier to tighten your screw down completely. Test the selector to make sure it clicks into both positions, safe and fire.

Step 7: Buffer Assembly

Step 7

Screw down the castle nut fully onto the buffer tube such that the notches would face the rear part of the buffer tube.

Move on to your end plate once you’re done calibrating the castle nut. You’ll notice an oval sticking out on one side and a corresponding shape dipping on the other.

Orient the side with protrusion facing the AR 15 lower receiver. You will observe some small protruding parts on the bottom.

Place the end plate into the narrow opening on the lower part of the buffer tube or receiver extension. Turn the tube into the back of the AR 15 lower receiver and make around 3 revolutions.

Once done, you’ll notice a large opening on the bottom of the threads on the back of the AR 15 lower receiver.  That hole is for the buffer retainer and its corresponding hole.

Installing the Buffer Retainer

  • Drop the buffer retainer and the corresponding spring into the hole.
  • Secure down the buffer retainer with your NON-DOMINANT THUMB while you proceed to twist the buffer tube in with your dominant thumb (NOTE: Do this until the buffer tube catches the very end of the buffer retainer
  • Check the buffer retainer for spring tension and turn the buffer tube clockwise just a few degrees until the top is around 30 degrees clockwise from the vertical or the 1 o’clock position.

After doing all the steps, you will see a small opening in the lower right part of the rear side of the AR 15 lower receiver.

The rear takedown pin detent and takedown detent spring will be placed in this opening.

Installing the Rear Takedown Pin Detent and Spring

  • Use a marker to mark the head of the rear takedown pin to search for the takedown detent channel.
  • Place the rear takedown pin through the right side and twist it so that the marking you’ve made faces the small hole in the rear takedown pin.
  • Drop the rear takedown detent in the hole and then drop the spring immediately.
  • Use a flat head screwdriver to cautiously push down the spring into the hole to avoid losing it.
  • Turn the buffer and end plate back into the appropriate position.

If the spring is bent, ensure that you have it straightened with pliers. Press down on the end plate until it fits snugly on the back of the AR 15 lower receiver.

Securing the Buffer Tube to the Receiver Extension

  • Secure the AR 15 lower receiver with your non-dominant thumb.
  • Twist the castle nut down over the top of the AR 15 lower receiver with your dominant hand. (NOTE: Do this until your connection between the buffer tube and your AR 15 lower receiver is hand tight.)
  • Tighten the castle nut completely using your castle nut wrench.
  • Put in a few light hits with your brass hammer on the wrench handle to adequately tighten the connection between the receiver extension and the AR 15 lower receiver.
  • Drop your buffer and buffer spring into the buffer tube or receiver extension.

After finishing these procedures, we proceed with the assembly of our fire control group:

Fire Control Group
  • Trigger
  • Hammer
  • Disconnector, Springs, and other related parts

Step 8: Trigger Assembly

Step 8

With your trigger assembly parts in hand, you can now proceed with the trigger assembly guide.

First, place the disconnector spring into the back part of the trigger. Then, install the trigger spring onto the trigger.

The longer legs of the trigger spring should be oriented towards the bottom of the trigger. On the other hand, the shorter legs of the trigger spring should be facing up.

The top part of the trigger spring, or the ends of the long legs, should sit snugly under the front ledge of the trigger.

Slowly shimmy the spring trigger on the trigger one side at a time over to where the trigger pin goes through.

Installing the Disconnector

  • Place the disconnector on top of the trigger so that the disconnector spring is located in the notch and the back of the disconnector.
  • Push on the disconnector down to see how the holes line up with the disconnector spring.
  • Temporarily remove the disconnector from the trigger for a while
  • Place the trigger into the lower receiver. You should see that the back of the trigger should have no problems sliding underneath the selector.
  • Drop the disconnector again onto the trigger and press down on the whole unit until all parts are properly lined up.

Inserting the Trigger Pin

  • You can now get your trigger pin and begin putting it through one part of the receiver.
  • Take a punch with the same diameter as the trigger pin and push it through the other end of the trigger pin hole until the punch and the pin meet.
  • Lightly push the pin all the way through using your hammer, preferably some small brass one. Doing so will help push the tool out the other end.

You shouldn’t have to hit the pin too hard if you have to use excessive force to try to get the pin through.

Step 9: Hammer Assembly

Step 9

Next, we now move on to the hammer assembly. It’s crucial to orient the hammer spring to the hammer.

In connecting the spring to the hammer, the short end of the spring should be facing the hook part of the hammer. The long end should be facing away from the hook part of the hammer.

Also, the legs should be facing the same side hook the hammer is facing. The hook and the legs should not be facing each other.

Install the spring onto the hammer in the said orientation over where the hammer pin goes through.

Make sure to shimmy it one side at a time. It should be similar to how you would attach the trigger spring to the trigger until it fits snugly.

Hammer Installation

  • Turn the hammer spring around until the top of the hammer is resting on the rear part of the hammer. (NOTE: The two legs of the hammer spring are going to push against the rear sides of the trigger where the trigger pin goes through.)
  • Place the legs of the hammer spring into the part mentioned above.
  • Push the bottom of the hammer into the proper location until the hammer pin holes line up with the hole in the lower receiver.

If your hammer doesn’t drop down low enough for your pin holes to line up, try placing the selector on fire. This should solve that problem, and your pin holes should line up after doing this.

  • Push your tool through the hammer pin hole on one side to secure the hammer in position.
  • Turn the receiver over.
  • Push your hammer pin through.
  • Lightly hit it into the pin hole.

Notes for Hammer Assembly

  • It is preferred that you use some small brass hammer or punch until it’s all the way through and flush with the receiver. These tools would make your life easier.
  • Again, if you have to hit your punch with excessive force, start over and try again. Check the trigger if it is functioning by testing the trigger on safe and fire.
  • Also, check the trigger reset by holding the trigger to the rear as you set the hammer back.
  • Don’t forget to block the hammer from jumping out with your finger since dry firing the trigger in this state without blocking it can cause damage to the receiver extension.

Step 10: Trigger Guard Assembly

Step 10

With your trigger guard assembly parts and tools in hand, you can now proceed to our guide with the trigger guard assembly.

  • Start by placing the front of the trigger guard into the lower receiver. This portion has a spring-loaded detent that has a pin secured in its place.
  • Orient the trigger guard so the end of the detent will easily connect into the hole on the right side.
  • Next, carefully push down the detent and slide the trigger guard up and between the two ears or tabs.
  • To support the ears, you can use a simple fixture from any gun manufacturer designer-author.

The fixture is specifically manufactured to support the ears or tabs while you drive in the roll pins.

Using a Fixture for Support

You can easily break the ears off if the ears are left unsupported. You cannot install your trigger guard if an ear or tab is broken.

All the tools in the world wouldn’t fix your trigger guard. Even a strong adhesive would probably not help.

You should always use a fixture to support the trigger guard. You want to waste multiple AR 15 lower receiver parts so that you can install a single trigger guard.

You must properly align the pin hole in the trigger guard with the holes in the ears. Start by using the roll pin starter punch. Afterward, switch to a standard roll pin punch.

A roll pin punch has a small protrusion on the tip that keeps it centered on the roll pin. This prevents the roll pin from slipping and damaging the finish.

Now drive the roll pin the rest of the way in. The pin should not protrude from either side.

Installing the trigger guard is fairly simple. Now you can see how important it is to support the receiver during this process.

AR 15 Lower Receiver Components

Components

The following parts are needed to assemble your very own AR 15 lower receiver:

  • Trigger and Trigger Spring
  • Hammer and Hammer Spring
  • Disconnector and Disconnector Spring
  • Trigger Pin and Hammer Pin
  • Bolt Catch with Roll Pin
  • Plunger (AKA Bolt Catch Buffer) and Spring
  • Magazine Catch Button, Magazine Catch, and Spring
  • Front Pivot Pin with Detent and Spring
  • Rear Takedown Pin with Detent and Spring
  • Safety selector with Detent and Spring
  • Pistol Grip Screw with Washer
  • Pistol Grip Buffer Tube (AKA Receiver Extension) with Retainer and Spring
  • Buffer and Buffer Spring
  • End Plate
  • Castle Nut

These parts usually come in kits so that you don’t have to buy the stuff one at a time. This is recommended for beginners since you are not yet customizing your AR 15 lower receiver.

Overview of the Upper Receiver

Overview

Although the upper receiver is an entirely different part of the AR 15 rifle, an expert author might think it’s beneficial to go over some of the parts of an upper receiver.

You can read our Guide on Building Your own AR15 Upper Receiver if you’re interested.

Upper Receiver

The upper receiver is the core part that contains the following:

  • Bolt carrier group
  • Charging handle

The following parts are also attached to the upper receiver:

  • Barrel
  • Forearm

Barrel

The barrel plays a huge role in your accuracy on the field or in the range.

Barrels come in different lengths and weights based on your personal priorities and preferences.

The model you choose will significantly affect the length of the gas system you will use on your AR 15.

Gas Block and Gas Tube

The AR 15 rifle design operates on a gas pressure mechanism.

Gas pressure forces the bolt carrier group into the buffer tube after you fire your AR 15. This process facilitates the ejection of a used round and the loading of a new one.

Upon firing your AR 15, gas moves behind the bullet that exits the barrel and moves through the gas port.

Afterward, the gas goes into the gas block below the gas tube and exits through the gas key of the bolt carrier.

In most cases, your barrel should be at least 16 inches long. Any shorter than that, and your weapon becomes illegal in most countries since it becomes a short-barreled rifle.

Carry Handle

Although it is not present in some AR 15 rifles, it was a staple in the typical author-expert early versions of the AR 15.

The carry handle increases the rear sight and allows an area to carry the rifle in balance and at the center of weight. Also, it was a common area in which you can mount a scope sight.

Where Do I Buy My Kits?

Where Do I Buy

Now that you have a guide and an overview of the process on how to build your very own AR 15 rifle, the next question you might ask is, where can you buy your own AR 15 rifle lower receiver kit.

Local Gun Store

A local gun author or enthusiast in your area might recommend this.

If you live in at least a medium-sized city, chances are you have already experienced buying your very own gun from your local gun store.

As the AR-15 rifle is quite popular, chances are, they will have the parts kit of the rifle on stock for you.

Any Benefits?

If you live in 1 of 40 US states which don’t have a waiting period for buying a gun, you might even be able to buy your parts kit right off the shelves.

You would be able to take a parts kit for a rifle just like you would buy any other product in your local general store.

This is the biggest benefit of going to your local gun store to buy the parts kit for your rifle. They are so accessible and convenient. You can immediately take home a rifle the same day you decide to buy one.

Amazing experience, right? But of course, just please always be a responsible gun owner. Keep your rifle, bullets, and your parts kit safely away from children so that they can be safe.

Any Downsides?

The downside is that if you have a specific product, customization, or specification in mind, your local gun store might not have the item in stock.

This might make you consider other options.

Online Gun Stores

Online gun stores remedy the downsides of going to your local gun store. Any specific brand or customization can be scoured through the comfort of your own home.

Also, if you have concerns or just questions in general about the products of the online gun stores, then you can just send out an email as you can easily get a hold of their email address.

A typical gun author outside your neighborhood and some gun enthusiasts may recommend this option for you to try out.

Of course, the downsides of these are the shipping fees and delays due to shipping. If you’re located quite far from the manufacturer, then you might have to wait weeks or even months to get your rifle.

You also might have to pay exorbitant shipping fees depending on where you are located.

If you’re a full-time penny pincher, then you might just have to watch out for free shipping promos!

Which AR 15 Parts Kit Should I Pick?

Which Parts

Well, this is a tough one. Typically, you should consult gun experts you know if you want the best products.

However, if there are none, you may be able to reach out to some famous gun expert, who usually is an author.

There certainly is some gun enthusiast and author who has written books about their knowledge.

For example, we have Patrick Sweeney, the author of “Gunsmithing the AR-15, Vol. 1: How to Maintain, Repair, and Accessorize”.

Reading the guide allows you to pick the author’s brain and develop some ideas in the process. You can also contact the author by reaching out to them.

The public email address of the author is usually accessible with a quick google search.

Final Words

The AR 15 Lower Receiver is the gold standard of modern-day rifles. Its popularity and modularity help it garner the title “America’s Rifle.”

Thanks to its modularity, you can be sure to have infinite options as to how to build and customize your AR 15.

It is now up to you on how to improve your AR 15 experience. We recommend reaching out to authors and receiver enthusiasts via their email addresses.

 

CHANGELOG:

September 13, 2021 - Reviewed and updated article links, updated article title

About the author

Christopher Wade

Christopher Wade is a true outdoorsman. After spending most of his career as a firearms expert and instructor in Nebraska, he retreated to the great outdoors to enjoy retirement.

Christopher’s expertise in handling firearms and hunting gear are what propelled him to create the Shooting Mystery blog. He hopes for all readers to gain useful and practical knowledge for enjoying their time outdoors.