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AR-15 Upper and Lower
The AR-15 comprises two receivers — the upper and the lower — attached using the front (pivot) and rear takedown pins. The upper receiver houses the bolt carrier group — the barrel/gas system connects to the upper. It also serves as a mounting platform for the sighting system, whether that consists of iron sights or optical sights.
The lower receiver houses the fire control group, magazine well, magazine catch, bolt catch, and selector lever. The lower receiver also contains information regarding the manufacturer, model, cartridge, and serial number.
For routine maintenance, such as cleaning and lubrication, you use the tip of a cartridge or other pointed tool to press out the rear takedown pin, allowing the lower receiver to pivot downward on the front takedown pin. When you’ve broken the gun open, you can remove the bolt carrier group and access the bore via the breech.
If you want to separate the two receivers, all you have to do is press out the front takedown pin and detach the two.
It's inevitable that you'll encounter problems while dealing with these parts of your rifle therefore it is preferable that you obtain knowledge on fixing the most common malfunction of AR-15s.
AR-15 Upper Assemblies
One of the advantages of the AR-15 platform is its modularity. The lower receiver is the legal firearm — it’s the base or foundation of the weapon system. You can swap upper receiver assemblies to change the caliber, barrel length, bolt carrier group, handguards, and other options.
Whether you choose to buy a complete AR-15 upper assembly or build one yourself, a federal firearms licensee is unnecessary. You can buy an upper receiver assembly online and have it shipped directly to your home. No ATF Form 4473 or NICS background check is required.
Upper Receiver Elements
When selecting an AR-15 upper assembly, there are various types and configurations to choose from.
The A1 and A2 upper receivers have integral carrying handles that house the rear sight assembly. In the A1 variant, the rear sight has a wheel for adjusting the windage. In the A2 variant, the dual-aperture rear sight is adjustable for windage and elevation.
Modern AR-15-pattern rifles feature a flat-top receiver, following the M4/M4A1 carbine, wherein a Picatinny accessory rail substitutes for the carrying handle. These types of receivers are the standard and offer increased flexibility for mounting sighting systems.
Due to the modularity of the AR-15 platform, you can change the caliber of your rifle by simply swapping out one upper receiver assembly for another while retaining the complete lower receiver — the legal firearm.
AR-15 upper receiver assemblies and rifles chambered in powerful cartridges, such as .450 Bushmaster, .458 SOCOM, and .50 Beowulf, are available for hunting, survival, and other specialized applications. The .458 SOCOM uses unmodified AR-15 magazines — ten rounds will fit in a 30-round USGI box. You may need to change the weight of the buffer to accommodate the increased bolt thrust in heavier calibers.
By changing upper receiver assemblies, you can also swap between barrels of different lengths, suitable for long-range precision target shooting or tactical training courses.
If you intend to use the direct-impingement gas system, there are different length gas systems depending on the barrel length. Each gas system has a different port distance from the front of the receiver.
However, not every AR-15-pattern rifle or upper receiver assembly features a traditional gas tube. Several examples exist on the market that uses a short-stroke gas piston with an operating rod that strikes a solid gas key on the bolt carrier.
Bolt Carrier Group
The bolt carrier group is the heart of the AR-15’s action. There are, broadly, two different types of bolt carrier: The M16 or full-auto bolt carrier and the civilian-legal variant.
The bolt, per military specification, is made from Carpenter 158 alloy steel; however, several companies manufacture bolts using 9310. The bolt carrier is made from 8620. These are proven materials, and the military specification sets the standard.
However, additional platings or coatings can reduce friction, increase lubricity, improve surface and wear resistance, and protect against corrosion. These include ferritic nitrocarburizing and electroless nickel-boron (NiB) are popular choices.
The traditional military handguards consist of a two-piece circular set held in place by the delta-ring assembly at the rear and the gas block in the front. These may be adequate for range practice or recreational shooting; however, they are not optimal for competitive target shooting, self-defense, and hunting.
Free-floating monolithic handguards are suspended solely by the barrel nut in the rear and do not contact the barrel or gas block, theoretically improving inherent accuracy.
Part of the reason for free-floating barreled actions is that the furniture of a rifle reacts to heat and environmental phenomena differently from the barrel. If the stock, for example, expands at a different rate or warps, it can cause the point of impact to shift.
Free-floating handguards are available in several configurations, depending on the type of rail or attachment system. If you’d prefer a cylindrical handguard, you’ll find examples fluted, knurled, or smooth. Quad-rail handguards used to be the standard but have become less popular over the years due to the emergence of the newer M-LOK and KeyMod systems.
M-LOK and KeyMod substitute a series of rectangular or keyhole-shaped slots to which you attach adapters, allowing you to mount flashlights, laser-aiming modules, and vertical foregrips.
The standard AR-15 charging handle is sufficient for most purposes; however, some shooters prefer to install a competition-ready ambidextrous charging handle. This type of charging handle allows you to more efficiently retract the handle with your left or right hand.
Manufacturers sometimes machine or engrave decorative designs into the outside of the upper receiver. This may be a matter of cosmetic preference, but they give you no tactical advantage whatsoever.
The forward assist is a spring-loaded plunger located on the right side of the upper receiver. Its purpose, when depressed, is to ensure that the bolt is fully forward, rotated, and locked. Whether the forward assist is needed remains a matter of debate.
Some manufacturers eliminate the forward assist from their upper receiver assemblies altogether. Whether you choose an upper receiver with one is a matter of personal preference.
Cerakoting and Other Paint Schemes
Finally, when choosing the upper receiver assembly that best complements your rifle, you can select from a variety of finishes. These range from Type III hard-coat anodizing, the black mil-spec finish on 7075-T6 aluminum, to various paints and coatings.
One of the most popular choices is Cerakote, a type of thin-film ceramic coating. Aside from wear resistance, Cerakote also allows you to apply various colors, patterns, and designs, personalizing your rifle.
If you want to camouflage your rifle temporarily, you can use a type of reusable fabric camouflage tape. In addition to applying the color scheme/pattern suitable to your environment, fabric tape can improve gripping texture, dampen sound, and add a non-reflective surface.
See Recent Article: Buying a Used AR-15, Is It Safe?
The Bottom Line
A good upper receiver assembly is the difference between a subpar rifle build and a state-of-the-art tack driver. Invest in the right parts and accessories for your needs, and your weapon will run like a champ.
While we're at it, upgrading your rifles is one of the most satisfying things to do with your firearm but would you prefer to include a laser on you AR-15? Find out here.