Tinkerers who build their own rifles are usually fairly familiar with the parts that constitute the upper and lower receiver groups.
For those who aren’t, learning what all the parts are (and what they do) can be somewhat intimidating. Most sporting rifles are fairly complex machines with a lot of moving parts.
Hopefully, this short guide will break down what the major parts of an AR 15 complete upper are, along with what they do.
The Upper Receiver (AKA Stripped Upper)
The upper receiver, also known as the stripped upper receiver, is the bare metal (aluminum) part that contains the bolt carrier group and accepts the charging handle. The barrel and rail system will also attach to the upper receiver.
If you’re not buying an AR 15 complete upper receiver, it helps to know what the differences are between the two main types, being forged and billet receivers.
A forged AR-15 upper is made from aluminum and hammered into the right shape and dimensions before being finished by machine. The forging process aligns the grain of the metal and ensures high uniformity, making forged uppers slightly stronger than billet uppers.
By contrast, a billet upper is machined from an extruded piece of barstock. Some billet uppers are more precisely machined than forged uppers, though both are finished by machining.
The Barrel
The barrel is another critical upper component that attaches to the front of the upper and is secured by an AR-15 barrel nut.
The barrel you choose is one of the most important decisions you can make with your build, as the quality of the barrel will impact barrel life, maneuverability, and accuracy.
Generally, longer barrels produce greater accuracy and more power, whereas shorter barrels create a lighter, more maneuverable barrel. Remember that your build must have a barrel of at least 16” or it will be classified as a “short-barreled rifle” which is an NFA item.
For long range shooting and precision shooting, consider a longer barrel, and for defensive applications, in most situations a shorter barrel length will be preferable.
Muzzle Devices
Some AR 15 complete upper assemblies come with muzzle devices. If you choose a threaded barrel, you will have to consider what sort of muzzle device you want to run.
There are three main types of muzzle devices, which are:
- Muzzle brakes: Muzzle brakes (called compensators on pistols) are special muzzle devices that redistribute gases at the muzzle so as to lessen the effect of felt recoil. On an AR-15, which is already a low-recoil platform (especially chambered in 5.56) a muzzle brake can almost entirely eliminate recoil.
- Flash hiders: Flash hiders help trap some of the gases escaping at the muzzle so as to conceal a firearm’s muzzle flash. This helps the user keep his or her position concealed.
- Suppressors (AKA Silencers): Suppressors, like muzzle brakes, trap some of the gases escaping at the muzzle. However, a brake makes a rifle much louder, whereas a suppressor makes a rifle much quieter. Be aware of two things, however: suppressors are rough on a gun’s action and will necessitate frequent thorough cleanings, and they are also NFA items.
Another note is that if your barrel is threaded and you don’t have a muzzle device installed, you should always keep the threads covered by a thread protector or they can easily be damaged.
The Handguard
The handguard, also known as the rail system, is the part that encloses the barrel and gives the user a place to grip the rifle, in the absence of a forend.
Sporting rifles have handguards that are covered by rail systems known as quad rails, in which there are rail slots above, below, and on both sides of the barrel. These can be used for attaching shooting accessories like scopes, red dot sights (and other optics) flip-up sights, sling attachment points, weapon-mounted lights, lasers, side saddle mounts, and more.
Three of the most common types of handguards are Picatinny, KeyMod, and M-LOK rail systems. Picatinny rails have been a standard for a long time, and so there are many attachments made for them. Two modern alternatives (both of which are lighter) are KeyMod and M-LOK rail systems.
The Bolt Carrier Group
The stripped upper receiver itself will contain the bolt carrier group, which is like the heart of the rifle, and which itself contains the gas rings, firing pin and spring, ejector, extractor, springs, and locking lugs.
The choice you make in a bolt carrier group is critical because there are all sorts of modern finishes (in addition to the traditional hard chrome and black phosphate coatings) which offer superior permanent dry lubricity.
For more information, check out our blog on bolt carrier group coatings. Also, keep in mind that not all AR 15 complete upper assemblies come with bolt carrier groups, so you may have to buy one even if you get a “complete” upper.
Gas System
AR-15 rifles use a gas impingement system, which relies on several parts working together. When you fire a round, gas travels behind the bullet, increasing in pressure. When it reaches the gas block (attached to the barrel) some of the gas gets siphoned off, through the gas tube, pushes the bolt carrier back (cycling the action), and exits through the gas key.
There are four main types of gas tubes; rifle, mid, carbine, and pistol length. The type you should use should be determined by the size and specs of your build. If you buy a complete upper, the gas system should already be installed and configured.
Charging Handle
The charging handle is the part that is used to manually charge the rifle and to clear jams. Most AR 15 complete upper assemblies come with charging handles but not all do so be aware of this when purchasing.
Forward Assist
The forward assist is a part you may never use, but its function is to manually force the bolt back into battery if for some reason the action doesn’t close all the way. Many AR 15 complete upper kits come with a forward assist but some do not, so keep on the lookout.
Ejection Port Cover
The ejection port cover is a basic and straightforward but important part that keeps dust and other particulate debris out of the open action when the rifle is in use. As with the charging handle and forward assist, not all complete upper assemblies come with them so be on the lookout when purchasing.
Beyond the AR 15 Complete Upper
These are the parts of an AR upper, broken down and explained by functionality. Remember that if you are building your own platform you will also need a lower receiver (the part that is serialized and regulated as the actual firearm) along with a complete lower parts kit, a stock, grip, and magazine.