AR-15 Components: A Complete Reference Guide

What This Article Covers

This guide maps every major component in an AR-15 build, organized by assembly. It explains what each part does, what’s inside upper and lower parts kits, and how the sub-components of the bolt carrier group work together. The goal is a single reference that gives new builders an accurate picture of the whole rifle before they begin selecting parts.

How the AR-15 Is Organized

The AR-15 divides into two major assemblies: the upper and the lower. The upper houses the barrel, gas system, handguard, and bolt carrier group. The lower houses the fire control group, magazine well, grip, and buffer system. The two halves connect via a pivot pin at the front and a takedown pin at the rear—no tools required.

The lower receiver is the serialized component, legally considered the firearm. The upper receiver is not serialized. This distinction matters for purchasing, transfers, and legal classification, but both halves are equally necessary for a functioning rifle.

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The Upper Assembly

Upper Receiver

The upper receiver is the structural backbone of the top half. It accepts the barrel, bolt carrier group, and charging handle, and attaches to the lower via the two receiver pins. Most uppers follow the mil-spec dimensions that define what handguards, barrels, and charging handles will fit. Variations exist—particularly for different calibers or proprietary handguard systems—so confirm compatibility before purchasing.

Barrel

The barrel determines caliber, legal configuration, effective range, and accuracy potential. Length affects velocity and maneuverability. Profile—pencil, government, heavy, or SOCOM—affects weight and heat tolerance during sustained fire. Twist rate Twist rate: the rate at which the barrel's rifling completes one full rotation, expressed as a ratio (e.g., 1:8). Faster twist rates stabilize heavier bullets. determines which bullet weights stabilize reliably. These decisions are interconnected: barrel length determines gas system length, which affects which gas block and gas tube you need.

For a detailed breakdown of these tradeoffs, see Choosing a Barrel: Length, Profile, and Twist Rate.

Handguard and Rail System

The handguard covers the barrel and provides the forward gripping surface and accessory mounting real estate. Free-floating handguards attach only at the barrel nut and make no contact with the barrel itself, which isolates the barrel from hand pressure and improves accuracy potential. Drop-in handguards contact the barrel nut at the front and the upper receiver at the rear, making them simpler to install but less rigid.

Rail mounting standards determine what accessories attach and how. M-LOK M-LOK: Magpul's slot-based mounting standard milled directly into the handguard surface. Accessories attach via T-nuts that slide into the slots. and KeyMod use recessed slots for a lower-profile attachment. Picatinny Picatinny (MIL-STD-1913): a standardized rail system of raised cross-slots that accepts a wide range of accessories via clamping saddles. rails provide the most universal accessory compatibility at the cost of added bulk and weight.

See Handguard and Rail Systems: M-LOK, Picatinny, Free-Float, and Drop-In for a full comparison.

Gas Block

The gas block sits over the barrel’s gas port and redirects propellant gas from the barrel into the gas tube. That gas drives the bolt carrier rearward to cycle the action. Gas blocks come in two main types: standard-profile (which sit above a traditional delta ring handguard system) and low-profile (which fit under free-floating handguards). Adjustable gas blocks add a metering screw or valve that controls how much gas enters the system—useful for suppressed builds or when tuning reliability across different ammunition.

Precise alignment with the barrel’s gas port is critical. An off-axis gas block will undergas or overgas the rifle regardless of other tuning.

Gas Tube

The gas tube is a hollow stainless steel tube that carries propellant gas from the gas block back to the bolt carrier key. Gas tube length must match the barrel’s gas system length: pistol, carbine, mid-length, or rifle. The length determines dwell time Dwell time: the amount of time the bullet remains in the barrel after it passes the gas port, during which gas pressure acts on the BCG. Longer dwell time means more gas energy delivered to the action.—how long gas acts on the bolt carrier—which directly influences reliability, bolt velocity, and felt recoil.

For a complete explanation of gas system lengths and their effects, see Understanding AR-15 Gas Systems.

Muzzle Device

The muzzle device threads onto the barrel’s muzzle end. The three main types serve distinct purposes: flash hiders reduce the visible signature of the muzzle blast; compensators redirect gas upward to limit muzzle rise during rapid fire; muzzle brakes redirect gas laterally to reduce felt recoil. A fourth category—suppressor mounts—includes direct-thread interfaces and QD mounts QD (Quick Detach) mount: a muzzle device designed to accept a suppressor via a rotational locking mechanism rather than threading, enabling fast attachment and removal. for attaching a suppressor.

Thread pitch must match the barrel’s threading. Most 5.56 barrels use 1/2×28 threads; .308 barrels typically use 5/8×24. A crush washer or shim set is required to time the device to the correct rotational position.

See Muzzle Devices: Flash Hiders, Compensators, and Brakes and Muzzle Devices for Suppressors for deeper coverage.

What’s Inside an Upper Parts Kit

Upper parts kits contain the small components needed to complete a stripped upper receiver. They are easy to overlook when budgeting a build, but a stripped upper without them is not functional.

• Forward assist: A spring-loaded pawl on the right side of the upper receiver. It engages serrations on the bolt carrier and allows the bolt to be pushed fully into battery manually if it fails to close under its own power.
• Ejection port cover (dust cover): A hinged cover over the ejection port that keeps debris out of the action. It opens automatically on the first cycle and can be closed manually when the rifle is slung or stored.
• Barrel nut: Threads onto the upper receiver and clamps the barrel extension in place. Free-floating handguards typically require a specific barrel nut—often included with the handguard—so confirm compatibility before purchasing an upper parts kit.

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The Bolt Carrier Group

The bolt carrier group BCG (Bolt Carrier Group): the self-contained assembly that chambers rounds, fires them, extracts spent casings, and cycles the action via gas pressure. is the operating heart of the rifle. It chambers rounds, fires them, extracts and ejects spent casings, and cycles the action. BCGs are sold as complete assemblies but consist of six interdependent parts:

• Bolt: The front section that locks into the barrel extension via rotating lugs. The bolt houses the extractor, ejector, and firing pin channel. Bolt material and shot peening Shot peening: a surface treatment that induces compressive stress to improve fatigue resistance. Common quality indicator on AR-15 bolts. are common quality indicators.
• Carrier: The outer body that the bolt rides inside. Gas enters through the gas key on top of the carrier, pushing it rearward. As the carrier travels back, its internal cam track rotates the bolt out of its locked position.
• Gas key: A small block staked to the top of the carrier that accepts gas from the gas tube and routes it into the carrier interior. Proper staking—where the key’s retaining screws are mechanically deformed to prevent loosening—is an important quality checkpoint.
• Firing pin: A hardened steel pin that travels through the carrier and strikes the cartridge primer. A small cross-pin (the firing pin retaining pin) keeps it captive in the carrier.
• Cam pin: A rectangular pin that rides in a cam track machined into the carrier. It converts the carrier’s linear rearward motion into bolt rotation, unlocking the bolt on the way back and locking it into battery on the way forward.
• Extractor: A spring-loaded claw at the bolt face that grips the cartridge case rim and pulls the spent case from the chamber as the carrier travels rearward.
• Ejector: A spring-loaded plunger also seated in the bolt face. As the spent case clears the chamber, the ejector pushes it away from the bolt and out the ejection port.

Full-auto carriers Full-auto (FA) carrier: a bolt carrier with a slightly larger lower surface than a semi-auto carrier. FA carriers are standard on most quality BCGs and are legal in semi-automatic rifles. are marginally heavier than semi-auto carriers and are standard on most quality BCGs regardless of the fire control group. BCG coatings—phosphate, nitride, nickel boron, and others—affect corrosion resistance, lubricity, and maintenance requirements.

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The Charging Handle

The charging handle sits in a channel at the rear of the upper receiver and allows the bolt carrier group to be cycled manually. Pulling it rearward retracts the BCG to chamber the first round, clear a malfunction, or perform a press check. Standard mil-spec charging handles use a narrow latch on the left side. Ambidextrous and extended-latch handles improve access for left-handed shooters, those running large optics with limited space behind the eyepiece, or anyone wearing gloves in cold weather.

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The Lower Assembly

Lower Receiver

The lower receiver is the serialized, federally regulated component—legally, it is the firearm. It houses the fire control group and magazine well, and provides the attachment point for the pistol grip, buffer tube, and upper receiver. Lower receivers are sold stripped (no internal parts) or complete (parts installed). A stripped lower requires a lower parts kit and trigger to function.

What’s Inside a Lower Parts Kit

A lower parts kit contains all the small components needed to complete a stripped lower receiver. Understanding what’s included clarifies why triggers are frequently sold separately and why kit quality affects both function and feel.

Fire Control Group

The fire control group Fire control group (FCG): the trigger, hammer, and disconnector assembly that controls when and how the rifle fires. consists of three parts working together:

• Trigger: The lever the shooter presses. It holds the hammer cocked via a sear surface and releases it when sufficient rearward pressure is applied.
• Hammer: A spring-driven component that strikes the firing pin when the trigger sear releases it. Hammer weight and spring tension affect primer ignition reliability.
• Disconnector: Catches the hammer on reset after firing, preventing it from following the bolt carrier forward before the trigger is released. The disconnector is what makes the rifle semi-automatic.
• Trigger pin and hammer pin: Cross-pins that retain the trigger and hammer in the lower receiver. Anti-walk pins prevent these from migrating during firing.

Controls

• Safety selector: Rotates between safe and fire positions. In the safe position, the selector’s geometry physically blocks the trigger from traveling far enough rearward to release the hammer.
• Magazine catch: The button on the right side of the receiver that releases the magazine. It consists of the catch body, button, and a spring.
• Bolt catch: A lever on the left side of the lower that locks the bolt carrier open on an empty magazine. When a fresh magazine is inserted and the bolt catch is depressed, the bolt carrier slams forward to chamber a round.

Pins and Small Parts

• Takedown pin and pivot pin: The two large pins that join the upper and lower receivers. Each requires a detent ball and spring to stay retained in the lower receiver.
• Buffer retainer and spring: A small plunger in the rear of the lower receiver that keeps the buffer captive in the buffer tube when the upper receiver is separated.
• Trigger guard: Protects the trigger from accidental engagement. Most mil-spec lowers use an integrated or pinned trigger guard.

Most standard LPKs include a mil-spec trigger group. Many builders replace the trigger with an aftermarket unit for a cleaner break and lighter pull while keeping the rest of the kit intact. See Trigger Selection: Single-Stage vs. Two-Stage for what to look for when evaluating upgrades.

Pistol Grip

The grip is the shooter’s primary contact point with the lower receiver. Grip angle, texture, and circumference affect comfort and trigger finger reach. Grips attach via a single screw and are among the simplest parts to swap. The grip screw retains the safety selector detent spring inside the lower receiver—a detail that catches first-time builders off guard during assembly.

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The Buffer System

The buffer system absorbs the bolt carrier’s rearward energy after firing and returns it forward to chamber the next round. Three parts work as a matched set.

Buffer Tube (Receiver Extension)

The buffer tube threads into the rear of the lower receiver and houses the buffer and action spring. It also provides the mounting surface for the stock or brace. Carbine-length tubes accept collapsible stocks; rifle-length tubes are used with fixed stocks. The two formats use different buffer lengths and springs and are not interchangeable.

Buffer

The buffer is the weighted cylinder that rides inside the tube and slows the bolt carrier’s rearward travel. Heavier buffers reduce bolt velocity and soften the recoil impulse but can cause short-stroking if the gas system doesn’t deliver enough energy. Standard designations—carbine, H, H2, H3—reflect increasing weight.

Buffer Spring (Action Spring)

The action spring returns the bolt carrier forward after the buffer compresses. Spring rate and length are matched to the buffer tube type. A worn or mismatched spring affects cycling consistency and lock-back reliability.

For a detailed breakdown of buffer system tuning, see The AR-15 Buffer System Explained.

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Stock or Brace

The stock mounts to the buffer tube and provides the rear contact point with the shooter. A proper stock allows a consistent cheek weld Cheek weld: the contact point between the shooter's cheek and the stock, which determines eye alignment with the optic or sights. for repeatable sight alignment. Adjustable stocks allow length of pull to be changed without tools, which matters for shooters using body armor or shooting from different positions. Pistol stabilizing braces mount the same way and were designed for single-handed use; their legal classification has changed over time and builders should verify current ATF guidance.

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Common Additions

These components are not required for the rifle to function but appear on most practical builds:

• Optic or iron sights: An aiming system. Iron sights are the baseline; optics extend capability. See Optics 101: Red Dots, LPVOs, and Magnified Scopes for an overview of the major categories.
• Optic mount: Attaches the optic to the upper receiver’s Picatinny rail. Mount height affects cheek weld and eye relief and is not a neutral choice. See Optic Mounting Height and Height Over Bore.
• Sling: Allows the rifle to be carried hands-free and can serve as a shooting stability aid. Single-point and two-point configurations handle differently.
• Weapon-mounted light: Essential for identifying targets in low-light conditions. Cannot shoot what cannot be identified.
• Foregrip: Attaches to the handguard to provide a forward gripping surface. Vertical grips have legal implications on short-barreled or pistol-configured rifles.

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This reference is intentionally broad rather than deep. Each section has dedicated guides that cover the tradeoffs in detail—start here to understand what each part does, then follow the linked guides when you’re ready to make decisions about specific components.