Introductory Guide to AR-15 Upper Receivers

For most civilians, our first contact with shoulder-held firearms comes in the form of traditional wood-stocked rifles. Typically, the wooden stock is attached with a metal receiver with everything attaches to it, including the barrel, trigger group and handguard.

However, with the introduction of the AR-15, one of the most popular rifles in use today, many traditional concepts changed, from the incorporated materials to the rifle configuration.

What’s a Gun’s Receiver, and Why Does an AR-15 Have Two

One of the distinguishing features from traditional guns is the AR-15’s modular design with multiple receivers, enabling it to be split into two distinctive and fully self-contained halves. Actually, this feature has persuaded shooters started calling the upper half the "upper receiver" and lower half the "lower receiver” for lack of a better term.

The bottom half, or lower receiver, contains all the parts needed to fire an AR-15 such as the trigger group, magazine well, buffer assembly, shoulder stock and pistol ​​grip.

According to the laws of many countries the gun receiver is the part of the firearm that carries the serial number. As such it’s the controlled component. That means it’s the part you need licensing for (where a gun license is required). In the case of the AR-15 the lower half is serialized and considered the legal receiver. The upper receiver doesn’t need to be licensed since it’s considered just a part and not the working core of the gun.

Components of an Upper Receiver

The upper receiver is the part that attaches to the AR-15 lower receiver and holds the barrel, sights, charging handle, handguard and bolt carrier group (BCG). Obviously, these are key parts of the gun.

Each of these parts can be replaced individually but they will be attached directly to the upper receiver, which in turn is attached to the lower receiver.

Assembly is relatively simple and requires few tools. We’ll come back to that in a bit.

Since the upper receiver is part of the gun’s skeleton, it includes features like a carry handle if you want one.

The reasons for the customizing are many. Buying a complete upper receiver allows you to quickly and easily replace some of the key parts of your gun. One advantage of swapping out AR-15 upper receivers instead of buying a complete rifle in the configuration you want surely is a price. While the factory AR-15 price tag is ranging from $800 to $1,500, complete upper assemblies can be found for anywhere from $300 to $900. If you want, you could have various upper receiver setups that you just need to swap out depending on how you’ll be using your gun on a particular day.

The term “complete upper receiver” sometimes can be a bit confusing because there’s no industry standard for what “complete” means. Some vendors offer a receiver with only a dust cover and forward assist installed. In other cases complete uppers may consist of the handguard, charging handle and even the BCG (bolt carrier group) and barrel. As earlier mentioned, you need to research manufacturer's ads thoroughly and check the full parts list of components to ensure you're getting everything you need.

A stripped upper receiver, on the other hand, does not include the forward assist or dust cover. You have to buy and add those yourself. It adds more complexity but also a higher degree of customization to your gun.

Installation of an Upper Receiver

The AR-15's split personality enables even novices to install a complete upper receiver assembly upgrade on any standard-sized lower AR-15 receiver in a simple “plug and play” manner. That means you can connect a dedicated varmint or precision upper assembly or swap out different uppers chambered in a variety of calibers that will work correctly with standard 5.56x45mm lowers.

However, more experienced AR-15 owners may not be satisfied with this type of semi-industrialized customization. They may want to have a fully-customized upper assembly because it’s so key for accuracy, ergonomics, and overall weight.

As with most modification or repair jobs on AR platform rifles, the installation of an upper receiver just requires simple manual skills and only a couple tools.  

For any kind of modification, you should first have a vice to hold your receiver in place. You’ll need a basic toolbox that includes a light hammer and snap ring pliers. There’s also a couple custom tools needed, including a quality clamshell upper receiver action block, an armorer's wrench (necessary for installation and removal of any barrel), a torque wrench and a small brass punch to knock out the gas tube roll pin.

Common sense of course goes a long way. Give yourself enough time to do the job right and work in a well-lit work area. Since some pins and springs are small, it’s handy to have a tray to keep them in and a clear workspace in case they get away from you.

Here are the basic steps you should follow in assembling and installing your new upper receiver:

  1. After removing the factory upper from the rifle, you should take a new upper receiver, place it into the vice block and install the FA (forward assist) and ejection port cover.

The FA tooth should be oriented toward the receiver and fixed with the roll pin. The next step will be the installation of the ejection port via the spring and its rod.

However, if you bought an assembled receiver, you will be spared from these two activities, and you can start with barrel mounting.

2. The new barrel can be easily installed into the receiver with the help of the index pin on the barrel end that fits in a groove in the upper receiver. The barrel nut is screwed just like any other. It should be torqued with your armorer wrench until the next gas tube hole aligns correctly so you can insert the gas tube.  For this operation, you can use a torque wrench, adequately set to 35 in-lb. (or according to specific gas block instructions). It allows for better seating between the surfaces when installing the barrel nut.

The next step depends on your particular handguard, but only sequence of steps may be different in this procedure.

3. After properly attaching the barrel, the next level is to insert the gas tube in gas block or FSB (Front Sight Base). Fix it with a roll pin and slip the tube over the barrel and through the barrel nut. When you install the gas tube into the gas block, the side with a bigger hole is going to face down the gas block.

4. Next, slip your new handguard over the barrel and thread it all the way to the rear to align perfectly and flush with the back edge of the barrel nut. If you are using a drop-in handguard, you can simply side it on. These types can be installed with almost no tools, and they do not require the removal of the barrel nut or the front sight block.

5. Put on the muzzle device with its large ports looking toward the side of the barrel, so that the majority of the gases are vented to the side.

6. Stick the charging handle part way in the small channel slot and then slide it about an inch forward.

7. Take the bolt carrier group with the bolt placed in its most-outward position and align the gas key on top into the channel on the bottom of the charging handle. Push it into the receiver until it locks into place.

8. Place the complete upper receiver onto the lower receiver and push in the takedown and pivot pins to connect the two halves. That easily, you have a different gun.

It’s a saying that dates to the Wild West days, but it’s equally valid today: “Beware the man with only one gun; he probably knows how to use it”. It’s appropriately used for the knowledgeable AR-15 owner, who can easily convert his one gun to have multiple purposes. Since the AR-15 is customizable for any purpose and has more purpose-driven parts, many people are now buying specific upper receivers to add flexibility.


Building a new upper receiver has many benefits. It allows you to remain comfortable with your lower parts like the trigger, buttstock, or grip. That way you’ll have the same stock configuration and trigger-pull all of the time. It also helps to save money.  It’s an easy exercise that allows you to add even more customization to your already-modular AR-15

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