The Ultimate Guide to AR-15 Lower Receivers

The lower receiver is considered to be the most important and primary part of a weapon. Federal law, customization capabilities and price of the AR-15 have all been an important factor in the story of aftermarket AR-15 lower receivers. Here we’ll be talking about the AR-15 lower receivers and their place within the rifle.

We’ll also talk about the different types and components of a lower receiver. You will also learn to install an AR-15 lower receiver to build up your weapon properly. So let’s get started.

What Makes a Gun a Gun?

In simple terms, the receiver of a weapon is the part which houses all other primary internal components (hammer, bolt and action). The receiver is also threaded to receive the barrel and the stock of the weapon.

According to firearms terminology and the law, the receiver itself is the firearm. According to the US law, a receiver has to be serialized and its sale and purchase are strictly regulated by law.

The main reason behind this is that the receiver is what combines all the essential working parts of a weapon and makes it capable to fire. The threads on the front and rear house the barrel and the stock. The base houses the magazine catch and trigger assembly, and the top houses the bolt, charging handle and ejection mechanism.

Talking specifically about the AR-15, this weapon has a unique receiver design. The AR-15 features a receiver which is divided into two parts, namely the ‘upper receiver’ and the ‘lower receiver’. 

This unique split design of the AR-15 makes it suitable for a number of aftermarket customizations related to aesthetics, functionalities and even caliber. More on that later.

The upper and lower receivers of the AR-15 are held by two pins which makes it almost effortless to separate the receivers for cleaning or customization.

In regards to the AR-15, the lower receiver is the part that has to bear a serial number and purchasing one requires an ID and a background check. An AR-15 lower receiver can only be sold by a licensed firearms dealer.

A complete or stripped lower receiver requires a federal firearms permit to purchase.

Buying all other parts of a weapon except the receiver is just like buying candy. No ID, no background check, no whatever. Just pay the price and you’re done. That’s because every other part of a weapon is practically useless without a receiver.

Components of a Lower Receiver

To state it in a single sentence, the lower receiver of an AR-15 contains the fire control group. The fire control group includes the trigger, disconnector, hammer, magazine catch and the fire selector.

Or in simpler terms, the lower receiver houses the trigger assembly and the magazine.

The lower receiver is secured to the upper receiver by two pins which can be easily removed for disassembly. This split design helps in customizing the upper receiver with ease.

If you are looking to create an entire lower receiver by yourself, you can buy an aftermarket lower parts kit for an AR-15 rifle. If you are using an unfinished receiver, you’ll have to machine all the slots to fit these parts in. If you are using a stripped one, all you have to do is simply drop in these parts.

The Czech MK3, a derivative of the ar15

A lower parts kit includes a magazine catch, bolt catch, pivot pin, fire control group, trigger guard, selector, grip, and pins. Further, you will need a complete rear stock and a spring assembly.

Complete vs Stripped vs 80% Lower Receivers

Basically, these three are the different stages of a receiver from start to end. These variants differ regarding federal regulations, price, customizability and ease of installation.

A complete AR-15 lower receiver, as the name suggests, is a finished ready-to-install product. It includes the trigger assembly, magazine catch, grip and sometimes the stock. You don’t have to put anything extra in it to make it work. All you have to do is simply attach it to the upper receiver and you’re ready to go.

Complete lower receivers require a federal firearms license before purchase. Since they are already ‘complete’, they are obviously more pricey than their counterparts.

A stripped AR-15 lower receiver is more in the middle stage of manufacturing. A stripped lower receiver has all the slots and holes precisely built in it using CNC machining. This means you can simply drop in your lower parts kit and you’ll have your lower receiver ready.

Stripped receivers are serialized. All the federal firearms laws and regulations as well as state laws and all other necessary paperwork apply.

An 80% receiver, also termed as ‘unfinished receiver’ or ‘blank’, is the main buzz nowadays.  An 80% receiver is an unfinished AR-15 lower receiver which does not require any serial number. That means that it is not regulated by the law. You can buy or sell one without any background checks, license or permits.

But this great advantage also has a drawback. The buyer needs to machine the receiver to make it usable. This requires proper machining and gunsmithing skills. An unfinished receiver is obviously inexpensive, customizable and doesn’t need a permit. However it still requires an expert as well as the proper machining equipment

Important Info

It is important to note here that a receiver you manufacture on your own from a blank does not need a serial number when it is finished. However, you are not allowed to sell, transfer or gift that weapon to anybody unless you apply for and get a serial number after proving it to the authorities that you manufactured the lower receiver yourself. Please consult a legal advisor before transferring or selling it.

Installation of a Lower Receiver

Okay, so here we’ll be digging deep into how to setup and install a lower receiver to the AR-15. This installation guide is primarily focused on installing stripped lower receivers. We’ll leave aside 80% receivers for now because they need to be machined first.

If you are looking to install a complete/finished receiver, please skip directly to step number 9.

Before we get started, let's first gather the tools and equipment we’ll be needing for this task:

  1. Stripped Lower Receiver
  2. AR-15 Lower Parts Kit
  3. Brass Pin Punch Set
  4. Rubber/Plastic ended hammer to push the pins without damaging the finish.
  5. C-clamp/Pliers to drive pins in.
  6. Hex-key set (preferably long arm)
  7. Utility Knife/blade or a Pen.
  8. Masking Tape
  9. Small Wooden Plank

Assembling an AR-15 lower receiver is a combination of several different steps. These include assembling the:

  • Trigger Guard Assembly
  • Magazine Catch Assembly
  • Bolt Catch Assembly
  • Trigger Assembly
  • Hammer Assembly
  • Selector and Pistol Grip Assembly
  • Buffer Tube Assembly

Step 1: Installing the Magazine Catch Assembly

Install the magazine catch in the wide hole on the left-hand side of the receiver. Insert the magazine catch spring from the right side of this hole.

Now place the magazine release button on the spring and screw the catch from the left side using a hex wrench.

Depress the magazine release button and further screw the catch so it fits flush into the assembly.

You can now insert a magazine and press the button to check if it works properly. 

Step 2: Installing the Trigger Guard

The next step is installing the trigger guard. Take the trigger guard and press the detent on the front part of the receiver base (just where the magazine well ends) so it snaps in.

Support the bottom flange of the trigger guard with a block of wood, rubber or plastic.

Now push the roll pin slowly in the rear of the trigger guard using the hammer and pin punch. Now you’re done with the trigger guard.

Step 3: Installing the Bolt Catch

Cover the area between selector switch and bolt catch with some tape to prevent any scratches.

Install the bolt catch retaining pin in the first hump. Now install the bolt catch spring into the hole on the left side of the receiver, and install the bolt catch plunger on top of it.

Now install the bolt catch in the receiver and drive the pin the rest of the way in.

Step 4: Installing the Front Takedown Pin

Insert the spring and the detent into the small hole of the receiver.

Compress the spring and the detent with a knife or the pens’ nib and slide the takedown pin into the hole.

Step 5: Installing the Trigger Assembly

Install the trigger spring onto the trigger with ends of the trigger spring facing forward and under the trigger.

Now push the disconnector spring inside the trigger all the way down.

Then position the disconnector on top of the spring and push it into the receiver. Now insert the trigger retaining pin using the hammer through the trigger, receiver and the disconnector.  

Step 6: Installing the Hammer Assembly

Put the spring on the hammer with ends of the hammer spring facing away from the hammer strike face.

Now insert the hammer into the receiver and punch to fit the hammer retaining pin into place. The ends of the hammer spring will rest on top of the top of the trigger pin.

Step 7: Installing the Safety Switch Selector and Grip

Cock the hammer back, hold it in place with your hand and install the selector switch through the hole, keeping it in the ‘safe’ position. Hold it and verify if the safety is working.

Now install the selector detent pin with the pointed end towards the selector and continue pushing the spring into the bottom hole of the receiver in the area where the grip is installed.

Insert the pistol grip screw with the washer inside the pistol grip and tighten it firmly to the receiver.

Step 8: Installing the Buffer Tube Assembly

Put the buffer retainer spring into the rear end of the receiver followed by the retainer. Now screw in the buffer tube and when it reaches the retainer pin, push the pin down using a screwdriver and screw the tube further so the retainer gets locked.

Insert the takedown pin detent with the point facing the takedown pin into the small hole on the backside of the receiver. Then insert the takedown pin behind it. Now tighten the castle nut to hold everything in place.

Install the buffer spring into the buffer tube and keep it in place using the retainer pin.

Step 9: Joining the Lower Receiver to the Upper

If you have a complete AR-15 receiver, you can jump directly to this step. Follow this step backward first if you need to disassemble the upper and lower receivers first, for replacing a complete lower receiver.  

Force the lower and upper receivers in place so they tightly fit in. Now push in the takedown pin and pivot pin and you’re good to go.

The takedown pin can be found on the upper part of the lower receiver just where the buttstock begins. The pivot pin is located on the upper front part of the lower receiver where the unit is pivoting.

Perform a function test by checking the chamber, checking the safety, check the trigger, cycle the chamber and check the trigger reset. If everything works perfect, congratulations, your AR-15 is ready to fire.


According to weapon terminology and the law, the receiver is itself the firearm. Federal laws require a receiver to be stamped with a serial number and its sale and transfer are regulated by law. The AR-15 features a unique split design being divided into upper and lower receivers.

The lower receiver holds the magazine catch, trigger, trigger guard, bolt catch assembly, selector, safety and the pistol grip. A lower receiver can either be complete, stripped or an 80% receiver. Due to the modular design, it is fairly easily to replace the lower receiver of an AR-15. Putting in just minimal effort and tools.  

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