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AR-15 Parts: The Builder's Master Guide

Posted by 80-lower on Nov 26th 2019

AR-15 Parts: The Builder's Master Guide

The AR-15 is arguably the most popular firearm bought and sold in America. You love the Second Amendment just as much as we do, and you want to own one. There are two ways to join the club and this guide's going to show you how to earn your membership: 

  • Option 1: Buy a mediocre AR-15 at a store that costs too much. Pay extra fees and taxes.
  • Option 2: Build a better AR-15. Avoid the bureaucracy by building it with an 80% lower.

You're still here, so we assume you like option 2 (the clear choice). 

This is your Master Guide for building an AR-15 from scratch, and it covers all the individual parts. Here we summarize every single component, its basic purpose, and how it functions. You'll find sub-guides linked throughout as you read. They dive into further detail about the AR's numerous systems: The gas system, barrel, lower receiver, parts kit, buffer system and weights, bolt carrier group, and more.

The AR platform's unique because it splits into two halves: The  upper receiver and the lower receiver. We'll start with the lower receiver assembly first -- it's how most shooters start this build project.


Lower Receiver Parts List

The lower receiver assembly is the "command center" of the rifle and consists of three systems of parts that work together to: Store and feed ammunition; Reduce and control recoil; Cycle the bolt carrier group; And operate the safety, trigger, hammer, and disconnector for semiautomatic fire. 

Those three systems are:

  1. Stripped lower receiver
  2. Lower parts kit and trigger
  3. Buffer assembly

1. The Stripped Lower Receiver

Building an AR-15 starts with buying or building a stripped lower receiver. The stripped lower is the only component of the weapon considered a firearm by law. All other parts of the AR-15 are are unregulated, bought and sold like any other product online or in a store. You have two options to pick from:

Option 1: Stripped Lower Receiver (firearm)

The stripped lower receiver is a ready-to-assemble firearm in the eyes of the law. Although the unit is not functional in any way (it doesn't have a barrel or upper receiver, and no trigger or buffer) you must go through the same legal paperwork and process when purchasing: Fill out a background check and paperwork, pay some fees to the FFL, and pay extra taxes.

Option 2: 80% Lower Receiver (blank)

Option 2 is why 80-Lower.com exists: You may instead fabricate your own stripped receiver at home to start your build by  picking up an 80% lower. This is a receiver blank not considered a firearm in the eyes of the ATF. Look closely and you'll see the main cavity for the trigger and parts kit is not hollowed out. It's also missing the holes for the hammer, trigger, and safety lever pins. Cutting and drilling the blank turns it into a stripped lower, a firearm. This is legal to do and is accomplished by using a gunsmithing tool called an 80% jig. The jig includes drill bits, an end mill bit, and instructions that show you how to get the job done.

This guide compares all 80% lowers and frames.


2. Lower Parts Kit & Trigger

AR-15 Lower Parts Kit Exploded Diagram

The lower parts kit -- primarily the fire control group -- performs the most important functions of the AR-15. It releases the hammer, striking the firing pine. It disconnects the hammer, allowing the trigger to reset. It allows the shooter to select "SAFE" or "FIRE", and it's responsible for catching and releasing the magazine and bolt.

Ready to assemble? Here's how to install the AR-15 lower parts kit.


Fire Control Group Components

The fire control group comprises the most important pieces of the parts kit: The hammer, hook, and hammer spring, trigger and trigger spring, disconnector and disconnector spring, and the trigger and hammer pins. These are all the parts that get installed directly inside the main cavity of the lower receiver. They’re responsible for performing all the functions listed above.

Hammer & Hammer Spring

The Hammer strikes the firing pin inside the bolt carrier group, causing a chambered cartridge (“round” or “bullet”) to fire. The hammer spring provides tension on the hammer when it is depressed down so that it slams forward, firing your rifle or pistol, when the trigger is pulled. The hammer springs rest on the trigger pin.

Hammer Hook

The hook is the portion of the hammer that interlocks with the disconnector after the BCG drives the hammer back down after firing. This keeps the hammer cocked temporarily until the trigger resets and the sear grabs the bottom of the hammer (where a cut-out for the sear is located) instead. When this occurs, the hook separates from the disconnector until the trigger is pulled again.

Trigger & Trigger Spring

The trigger is the “fun button" that releases the hammer when cocked, allowing it fly up and strike the firing pin, igniting a round in the firing chamber. The trigger assembly comprises the trigger spring (pictured above, wrapped around the trigger itself), the top-rear shelf for the disconnector spring, and the sear. The sear is the small bar above and in front of the trigger and is arguably the most important part of the trigger. 

As you squeeze the trigger after fire a shot, the hammer flies up to strike the firing pin. The bolt carrier group rides backward, forcing the hammer back down. Like we said, the disconnector initially grabs the hammer while the trigger is still being squeezed. As the trigger is released to reset to the firing position, the disconnector disengages the hammer and "hands off" the job of holding the cocked hammer to the sear.

Disconnector & Disconnector Spring

The disconnector sits atop the trigger and attaches to the trigger assembly using the trigger pin. As you now know, the disconnector catches the hammer after the bolt carrier group rides backwards, holding it down until the sear re-engages the hook on the hammer. The disconnector spring (and the movement of the trigger compressing the spring) provides the necessary tension for the disconnector to do its job and grab the hammer.

Hammer & Trigger Pins

The hammer and trigger pins are responsible for keeping the entire fire control group secured and moving as one precisely-tuned unit. To ensure the hammer and trigger springs remain seated (and to help prevent the pins from "walking out" of their holes in the receiver), each pin has small notches for the legs of the springs to be seated upon.

How the fire control group functions

Visualizing all this can be confusing. Let's recap how the fire control group parts all works together in one, fluid motion by seeing it in action:

Follow along with the animation: The trigger is pulled, releasing the sear from the hammer. The hammer strikes the firing pin inside the BCG. The round fires and the BCG is forced backward. This pushes the hammer back down. The trigger isn't reset yet, so the disconnector automatically catches the hook on the hammer. The shooter lets off the trigger, resetting it. The trigger's sear again catches the hammer to keep it cocked, releasing the disconnector from the hook. The hammer's re-cocked and a new round is ready to fire.


Remaining LPK Components

The remaining parts of the LPK include all the other pieces you'll need to turn your stripped lower receiver into a complete, ready-to-fire lower assembly:

Selector Lever (“Safety”)

The selector switch, commonly called the “safety”, is the lever that sits flush against the left side of the lower receiver. When engaged, the rounded portion of the lever inside the receiver prevents the trigger from being pulled. When disengaged, the selector switch rotates, exposing a flattened portion of metal that allows enough clearance for the trigger to be pulled. A small detent and spring capture the lever, preventing it from falling out of the receiver.

Magazine Catch & Button

Shown assembled together, the magazine catch and button work together, allowing you to insert and remove magazines. The notched portion of the catch engages with the magazine inside the well, trapping it in the receiver. When the button is depressed, the catch is released and the magazine falls. The spring keeps constant tension against the button, preventing the magazine from falling out when until button is not depressed.

Takedown & Pivot Pins

The pivot and takedown pins keep the AR's upper and lower receiver assemblies secured to each other. The pins are permanently installed at the front and rear of the lower receiver, trapped in place by spring-loaded detents. Pulling on the rear takedown pin allows one to access the fire control group and BCG for quick cleaning and troubleshooting. Pulling the front pivot pin allows for complete separation of the upper and lower receivers.

Bolt Catch Assembly

As the name implies, the bolt catch grabs the bolt carrier group when it's out of battery, preventing it from riding forward. To do this, the shooter must press on the bottom of the catch. The textured button face releases the bolt, driving it forward and chambering a round if a magazine is loaded. A spring, buffer, and roll pin secure the catch and provide tension against the button, preventing the catch from grabbing the BCG unless the shooter presses it.

Buffer Retainer

The buffer retainer rests at the rear of the lower receiver, inside the buffer tube housing and threads. Its job is to prevent the recoil spring and buffer from decompressing and flying out of the tube each time the receivers are separated. The small nib atop the retainer rests against the face of the buffer. To remove the buffer, simply press down on the retainer.

Detents and Springs

A series of detents and springs are used to secure some of the moving parts to the receiver, like the pivot and takedown pins, and the safety selector lever. These spring-loaded detents act like little "stops", preventing the parts from falling out when pulled or rotated. Look at the pins above again, and you'll see the cut channels where the detents rest.


3. Buffer Assembly

The buffer assembly consists of the recoil spring, weighted buffer, receiver extension (buffer tube), buttstock, latch plate/sling mount, and the castle nut for tightening the tube to the lower receiver.

Receiver Extension (Buffer Tube)

The receiver extension (called a buffer tube by most) is responsible for housing the recoil spring and buffer. When the AR-15 is fired and the bolt carrier group is forced backward, it rides into the receiver extension and compresses the recoil spring behind the buffer.

Recoil Spring

You already know the job of the recoil spring. There are two common kinds of springs for the AR-15, one being designed for the longer rifle receiver extension with a fixed A2 stock (measuring 12"), and the shorter 10" carbine spring, designed for the adjustable buttstock.

Buffer

The buffer is responsible for handling the recoil and blunt force created by the rapid movement of the BCG. The flat face of the buffer rests against the backside of the BCG, while the pointed end with the rubber cap rests inside the recoil spring. The rubber end is designed to eliminate damage to the buffer tube if it bottoms out under excessive recoil.

Castle Nut & Latch Plate

The castle nut tightens the receiver extension to the back of the lower receiver, while the latch plate prevents the extension/tube from rotating in the threads. The small "nib" on the inside ring of the latch plate locks into the channel cut underneath the buffer tube. The round protrusion on the bottom of the plate rests inside an indentation at the back of the receiver, trapping the plate in place.

Buttstock

Self-explanatory, the buttstock is your shoulder buddy. It allows you to obtain a good sight picture and enjoy stability with your rifle while you land rounds downrange. The AR-15's platform generally uses two configurations (again fixed, or adjustable) with the latter being the most common by a wide margin. Shown above is how the castle nut and latch plate are installed to secure the tube and buttstock to the back of the lower receiver.

Pistol Brace

You've probably seen an AR pistol or two by now. Some choose to build compact shooters with a barrel shorter than 16". Since this is the minimum legal length for a barrel on a rifle, you must ditch the buttstock or you risk the law coming down on you for illegally building a short-barreled rifle (an NFA item). However, the pistol brace makes for a great alternative. 

Building an AR pistol? Read our guide to pistol braces here.


Upper Receiver Parts List

Take a break and grab a coffee (or beer, it's 5 o' clock somewhere) because we're half-way through our master list of AR-15 parts. The rifle's upper receiver assembly constitutes the "business end" of things: It's where the firing pin meets the primer and live rounds go "boom" down the barrel. Like the lower receiver, the upper consists of systems of parts that work together:

  1. The impingement gas system
  2. The stripped upper receiver
  3. The bolt carrier group
  4. The handguard
  5. The barrel 

1. The Gas System

The AR-15's gas system is a direct-impingement system: Gas expelled by the cartridge fired in the upper receiver travels down the barrel, behind the projectile. It meets the gas system, which forces part of the gas through a tube and into the upper receiver, where it meets the bolt carrier group, forcing it into the buffer assembly. The gas system combines the following components:

Gas Block

The gas block secures directly to the far end of the barrel, near the muzzle. A small hole is drilled in the barrel to act as a port for the gas to travel to the block. most gas ports have a diameter of 0.063" to 0.089", depending on barrel length. Gas blocks are most commonly found in two configurations shown above: The classic "A2-style" front sight post and gas block combo (pictured left), and the modern low-profile block (pictured right).

Gas Tube

The gas tube connects the gas block to a port in the front of the upper receiver, located directly above the barrel nut. Part of the tube protrudes through the port and into the receiver itself, where it interfaces with the gas key atop the bolt carrier group. Tube length is dictated by selecting one of the four available gas systems:

  • Pistol (6.75" tube)
  • Carbine (9.75" tube)
  • Mid-length (11.75" tube)
  • Rifle (15.25" tube)

BCG Gas Key

Although it's a component of the bolt carrier group, the gas key is part of the gas system, too. The key is hollow, allowing gas from the gas tube to be directed into the body of the BCG. This expansion of gas forces the carrier backwards into the buffer tube. This cycles the weapon by ejecting and chambering another round, and resetting the trigger, hammer, and disconnector.

Ready to build the perfect setup? Read our complete Builder's Guide to AR Gas Systems.


2. The Upper Receiver

The stripped upper receiver is responsible for securing the barrel and barrel extension (firing chamber) so live rounds can be fed to the chamber and ignited. It contains the bolt carrier group while in battery, positioning it so the hammer can release and strike the firing pin when the trigger is pulled. Pictured above, the stripped upper receiver is incredibly simple by itself: The only components of note are the dust cover and forward assist. Both features are optional and aren't required for the weapon to function.


3. The Bolt Carrier Group

The  bolt carrier group (BCG) rests inside the stripped upper receiver. The BCG is responsible for igniting live rounds with the firing pin and hammer. The BCG is one of the most important parts of the AR-15.

Read the full guide to the AR Bolt Carrier Group here.

4. The Handguard

The handguard secures to the upper receiver and, in some cases, the barrel. It's responsible for providing the shooter with a comfortable forward grip, protecting bare skin from the heat generated by live rounds and gas. Exampled in our photos is a typical "A2" handguard, styled after the service rifles used by the military in prior decades. This type of handguard also secures to the back of the front sight post, shown below. Other handguards have been introduced with new attachment systems like Picatinny, M-Lok, and Keymod. Many now install as "free-float" handguards: They don't secure to the barrel in any way, just the upper receiver. This reduces pressure on the barrel and increases accuracy.


5. The Barrel

Last, but certainly far from least: The barrel is the business of the operation, and it's the most important part of the entire AR-15. This example is perhaps the most common barrel you'll find on a rifle: It measures 16", is made from 4150 Chromoly Vanadium steel and features a 1:7 twist rate. It hosts a carbine-length gas system and is coated with a Melonite finish. 

Learn more: AR-15 Barrels Explained (Barrel & Steel Types + Coatings)

Important AR-15 Barrel Specifications

Length. 

Length dictates how much physical barrel you're working with. The longer the barrel, the higher the velocity of the fired round exiting the muzzle. Most AR-15 barrels are 16". 

Twist rate. 

Twist rate measures how often the rifling inside the barrel makes one full rotation, measured in inches. A 1:7 twist rate completes one full rotation in seven inches. A 1:8 twist rate, eight inches. The heavier the bullet being fired, the lower (faster) the twist rate. Most AR-15's use a 1:7 or 1:8 twist.

Barrel contour. 

Contour defines the exterior shape of the barrel. The barrel shown sports the common "M4" or "Government" profile, thinning behind the gas port, then getting wider toward the muzzle. Custom barrels might have fluting or dimpling to reduce weight while stiffening the barrel. Others barrels may sport a single, thick diameter from end to end - these are usually called "bull barrels".

Gas block diameter. 

We covered this above: Each barrel will have the gas block diameter described. 

Gas system length. 

Self-explanatory, again: The barrel's gas system will be described as pistol, carbine, mid-length, or rifle.

Feed ramps. 

The feed ramps are responsible for guiding rounds from the magazine into the chamber. Higher-end barrels use "M4" feed ramps, which are mil-spec ramps considered to be the most reliable for 5.56 NATO and .223 Remington.

Interior lining. 

Barrels can be lined with chrome, greatly enhancing their lifespan and ability to withstand heat from rapid fire. Others (like stainless barrels) may be un-lined, which provides better accuracy. Today, many AR-15 barrels are coated with Nitride (also called Melonite), which provides the benefits of chrome lining without a loss in accuracy. 

Exterior coating. 

Non-stainless barrels are coated on the exterior to protect against rust and corrosion. The most common exterior coating is also Nitride/Melonite, or phosphate if the interior is already chrome-lined.


Helpful Guides & Other AR Builds

Already have a warm n' fuzzy about the typical AR platform? This guide focused on the parts required to build a traditional, mil-spec AR-15 chambered in 5.56 NATO (or .223 Remington). Here are some helpful guides for completing your 80% lower, rifle, or pistol, as well as guides for other types of builds:

DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At 80-lower.com, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.