What is an Upper Receiver? [The History & Different Types]

If you’re new to the world of AR-15s and firearms, there’s a lot of terminology to become accustomed to.

One of those keywords you’ll hear a lot of talk about is the upper receiver.

The quality of an AR-15’s upper receiver is integral to its function, accuracy, and modularity, which ensures the firearm’s reliability in cycling ammunition efficiently, while maintaining shot precision. Machined with aluminum or polymer materials, the upper receiver is a key component  of the AR because it houses the BCG, connects the barrel, handguard, and the railed mounting system to the firearm’s lower receiver. 

So, what is an upper receiver? Basically, the upper receiver serves as the central hub to the rifle’s various critical components and operations.

Now, let’s take a closer look at the upper receiver’s different functions.

The Upper Receiver’s Housing and Vital Components

The upper receiver houses and protects several of the firearm’s most crucial innermost parts, which include the bolt carrier group (BCG) and the charging handle.

The BCG is fundamental to the rifle’s operation and framework because – in conjunction with the charging handle – it  facilitates the cycling through of ammunition, while clearing any potential jams.   

What’s more, the BCG, with the support of the mounted charging handle, expels spent cartridges through the ejection port, resets the hammer, and chambers a new round, which readies the firearm for another shot.

Barrel Integration with the AR Upper

The connection between the upper receiver and the barrel ensures that the latter is aligned correctly, which is vital in terms of the firearm’s accuracy. The barrel’s length and twist rate – how quickly the beveled rifling inside the barrel spins the bullet – significantly influence the bullet’s velocity and stabilization once fired. 

This phenomenon not only affects how accurate a projectile’s trajectory is, but it greatly impacts the firearm’s overall range, as well.

Optics and Accessory Mounting Systems of the AR Upper

The manufacturer-milled upper receivers of today often see Picatinny, M-LOK, or KeyMod rails mounted to its top side. These straightforward rail systems allow operators to customize their AR-15s with various optics, including red-dot sights, holographic sights, and downrange scopes.

Furthermore, certain uppers can fasten rails to their sides and bottoms to facilitate the attachment of accessories such as flashlights, laser sights, and forward grips. 

Such customizable features only underpins the AR’s adaptability to undertake diverse shooting scenarios – from close quarter engagements to long-range shooting.

Upper Receiver Ejection Port and Gas System

The ejection port, as its name suggests, expels spent bullet casings once a round has been fired.

Above this port is the forward assist, which is a mechanized plunger-like system used to prevent jamming by effectively re-seating and cycling through an improperly-seated round. This firing chamber safeguard ensures that the firearm remains in battery, which means that a round has been chambered and the gun is safe to fire. 

The AR-15 utilizes a direct impingement gas or a piston-operated gas recoil system. 

In terms of the gas impingement system, gas from the fired round travels through a port in the barrel, to a gas tube, and into the BCG. This remaining gas expands and drives the BCG backward, expelling the spent bullet casing. The spring and buffer compress inside the buffer tube, jolting the BCG forward into the upper receiver. As the BCG travels forward, it grabs another round from the magazine well and chambers it, making it ready to fire.

The piston-operated gas system – first utilized in the development of the AK-47 – works similarly to its gas impingement counterpart. But, instead of gas being bled into the tube from the barrel, it’s diverted into a separate cylinder containing the piston and expands. Unlike with the impingement gas system, the piston is the driving force to cycle the BCG.

Integration with the AR’s Lower Receiver

The upper receiver attaches to the lower, which houses the trigger group, magazine well, and the stock. This modular design is standard with the AR platform and allows operators to quickly and easily switch out upper receivers. What’s more, shooters can customize their uppers without requiring an entirely new AR build.

That said, mounting a new upper can present compatibility issues, especially when integrating aftermarket components and/or exotic (non-standardized) parts to an AR build. What’s more, swapping out uppers has the potential to change the firearm’s caliber, barrel length, and configuration, which are certainly things to consider when deciding to upgrade from a manufacturer’s standard upper to an out-of-the-box URG.

Service Rifle Upper Receiver Types

The wartime AR upper receiver has a storied development history from the deployment of the original A1 during the Vietnam War to the more modern design advancements of today. Let’s look at the different upper types.

The A1

The original upper receiver was configured and mounted to the M16A1, which was adopted during the Vietnam War for its simple mechanics and durable aluminum build. This receiver has a fixed carry handle as well as a forward assist. The upper’s rear sight is adjustable to account for windage, but not for elevation.

The A2

Designed as an upgraded successor to the A1 model, the A2 features a fixed carry handle, but now includes an adjustable rear sight that accounts for both windage and elevation. The elevation sight component was absent on the A1. Moreover, the forward assist and lefty-friendly brass deflector ejection port module comes standard.

The A3 and A4

Often referred to as “flat tops,” these two designs do away with the fixed carry handle. Instead the A3 has a detachable carry handle with Picatinny rails underneath. The A4 model, on the other hand, comes standard with a Picatinny rail system on its top side. The rail systems on either design allows for easy mounting of optics and sights. 

The A3 is configured in semi-auto, while the A4 variant has a select-fire setting, which can be toggled between single and semi-auto fire. 

Billet

These uppers are milled via CNC-machining from a single block of aluminum known as a billet. This receiver-type is set apart because they are designed with unique aesthetic touches. Furthermore, billet uppers feature additional milled refinements and design characteristics, such as unified trigger guards or larger ejection ports, which are not typically found on standard forged receivers. 

Forged

Forged uppers are crafted by hammering two halves of raw aluminum into a die/mold and forged under high pressure. The molded metal undergoes further machining to refine its shape to ensure a proper fit to an AR’s build, while adding a level of durability not seen in other manufacturing processes.

Polymer

These uppers are made from reinforced polymer plastic materials and are more lightweight than those made with aluminum. Polymer receivers are also more cost-effective and offer more design customization than that of their competition. What’s more, unlike their aluminum counterparts, polymer receivers are resistant to rust and corrosion that typical gun metals are so often subject to.

While A1/A2/A3/A4 nomenclatures have been designated for M16 service rifle-types during wartime, many of the military-grade firearm developments implemented during that time were adopted and further enhanced with the introduction of the AR platform. But it was these initial advancements that paved the way for the modern rifle technologies we have today. 

Stripped vs. Complete Upper Receivers

When it comes to mounting upper receivers to an AR build, there are a plethora of aftermarket options available. But, every operator’s preferences are different depending on their intended vision for their weapon – from its features to its mounted gun systems. There are shooters who appreciate a more labor-intensive approach to their builds, while others want to attach a ‘no muss, no fuss’ upper and hit the range. 

With that in mind, let’s look at stripped and complete upper receivers.

Stripped Receivers

A stripped receiver, as its name suggests, consists of the receiver’s shell and that’s about it. These DIY uppers are devoid of the BCG, the barrel, charging handle, forward assist, or handguard, which come standard with completed receivers. This model does allow for greater customization possibilities for shooters, but all other necessary components must be researched and sourced separately.

Complete Receivers  

Complete receivers are fully assembled aftermarket uppers that are equipped with the barrel, BCG, handguard, and in certain models, manufactured sights are included. These assembled receivers offer a more uniform solution for operators that don’t wish to subject themselves to the potential headaches that come with sourcing individual components and a moderate level of gunsmithing knowledge.

Final Thoughts on What is an Upper Receiver?

You should now have a working understanding of what the upper receiver is.

With that being said, nothing compares to the experience and understanding you gain from holding a rifle and examining its components firsthand. By physically interacting with an upper receiver, you can truly appreciate its design, functionality, and the role it plays in the overall performance of a firearm.

Whether you’re assembling a new build or handling your first AR-15, handling the parts allows you to see how each piece fits together and operates.

Justin Trump

Justin Trump is the managing editor and owner of CAT Outdoors. The son of a Vietnam veteran, he’s an avid gun enthusiast and 2A advocate. He holds two firearm patents for the CAT M4 and Talon tools. When not managing CAT Outdoors, he enjoys spending time with his family and friends, rooting for Michigan sports teams, and serving his church.