This guide covers the parts required for building your own AR-15 at home, legally. We'll cover the available setups, including calibers and configurations. We'll also touch on ballistics, performance, and reliability as we discuss each major component, so you have the knowledge to customize your rifle or pistol and choose the parts you want confidently. First, there are some questions and answers to learn.
Building an AR-15: Frequent Questions
Q: Is it legal to build an AR-15 at home?
Yes. You often see kits for sale because they are legal to purchase, transport, and own. the Gun Control Act of 1968 also says it's legal to build and assemble a gun at home.
Q: Do I need a background check to build my AR-15?
That depends. You can purchase a stripped lower receiver which is considered a firearm. If you use a stripped lower receiver to build your AR, you'll need to purchase it through an FFL (Federal Firearms Licensee) dealer. That means filling out some paperwork and paying some fees for a background check. There may be a wait period depending on your state.
The other option is an 80% lower receiver. This is a firearm blank, a component not considered a firearm under federal law. The 80% lower must be fabricated by the end user using tools to make it a functional firearm. This is also covered under the Gun Control Act and is legal to do in most states. You do not need to visit an FFL or conduct a background check if you purchase and fabricate an 80% lower.
Q: What about buying all these parts online?
That's legal. Since no parts of the AR-15 besides the lower receiver are considered a firearm, those parts can be purchased online and shipped to your house, just like any other standard commercial or consumer product.
Q: How much is my build going to cost?
That depends. The AR parts market is utterly flooded with manufacturers. That means you can get a high-tier kit or components that aren't relatively expensive. A "mil-spec" kit that includes everything you need (minus the lower) and is capable of shooting 1" groups at 100 yards can be bought for around $500. Top-tier barrels and match-grade parts can total up to over $1,000, though most builders see diminishing returns on their investment around the $500 to $750 price point. Even "entry-level" parts available today are relatively high in quality thanks to stiff competition.
Q: How easy is it to build an AR-15?
This project requires very few tools and no gunsmithing knowledge. You'll need to assemble the lower receiver with your trigger and parts kit. We wrote a guide for this installation with plenty of pictures to walk you through step-by-step. All other parts, including the upper receiver assembly, attach without tools.
Build Summary: The Parts You Need
The AR-15 is broken into two halves: The upper receiver, and the lower receiver. Each half can be purchased as a fully assembled unit. Virtually all first-time builders purchase the upper receiver assembled, while assembling the lower receiver themselves. Each half contains the following parts:
Upper Receiver Assembly
- Gas tube
- Gas block
- Charging handle
- Bolt carrier group
- Stripped upper receiver
Lower Receiver Assembly
- Buffer tube
- Recoil Buffer
- Recoil spring
- Lower parts kit (hammer, trigger, disconnector, grip, magazine and bolt catch/release, and safety)
Let's break down each half and the individual components. We'll give a brief summary of each part and what specifications are important. Since most builders choose the 5.56 NATO cartridge for their first build, we'll use that for our example parts below. At the bottom of this guide we'll briefly cover the "optimal parts setup" for 9mm (AR9), 300 Blackout (AR-15), and .308 Winchester (AR-10).
Upper Receiver Parts
The barrel is arguably the most important component of your AR-15 build. It determines what caliber you'll shoot, how far you can shoot accurately, how long your gas system is, and how long your rifle or pistol measures. Consider these attributes carefully:
Twist rate defines how often the bullet twists in the barrel before exiting. This is measured by the number of inches per rotation. A 1:7 twist means you get one rotation every seven inches. The chart below illustrates the best twist rates depending on your choice of ammo.
Virtually all store-bought ammunition is 55-grain or 62-grain. That's why most builders stick with a 1:7 to 1:9 twist. The 1:8 twist is a perfect compromise and is best suited for a wide range of ammo.
The minimum legal length a rifle's barrel must be is 16". Thankfully, 16" is the optimal barrel length for achieving good velocity and stability with 5.56 NATO. For AR pistol builds, shooters say the minimum length for 5.56 is 7.5". Below this length, velocities are typically too low to provide favorable accuracy at distances beyond 50 yards. Here's a more in-depth guide discussing barrel length, twist rate, and velocity.
Virtually all barrels are made from "mil-spec" chromoly-vanadium (CMV) or 4150 steel with Vanadium added. If you opt for a stainless barrel -- which typically provides slightly better accuracy compared to a carbon steel barrel -- you'll want to stick with 416R stainless. It's the only stainless steel gauranteed to operate safely in hot and sub-zero freezing temperatures.
The two most common coatings are phospate (which is mil-spec) or nitride. Phosphate is a spray coating while nitride is a chemically-based coating that gets "baked" into the interior and exterior of the barrel. Both appear flat black. We prefer nitride, though if you go with a phosphate barrel, you won't notice any appreciable difference in accuracy or barrel life. Phosphate doesn't coat the rifling inside the barrel, but nitride does.
Handguards connect to the upper receiver with two methods: Free-float, where the handguard attaches via the barrel nut and "floats" over the barrel, and two-piece handguards. Two-piece handguards attach to the barrel nut and the barrel itself, usually requiring an A2-style front sight. Two-piece handguards are tougher and offer the classic "M16" look, while free-float guards provide more accuracy and are typically lighter and more expensive.
Most AR kits include free-float handguards. We recommend avoiding KeyMod and sticking with the M-Lok or Picatinny rail and accessory attachment systems. Free-float handguards require a low-profile gas block, which a perfect segue to our next topic:
The Gas System
The gas system consists of a gas block and tube. The gas block connects to the barrel and diverts gas from the barrel back into the upper receiver via the gas tube. Together, this system provides the energy required for the bolt carrier group to cycle inside the upper receiver, ejecting a spent round and chambering a new round while resetting the hammer and trigger.
The AR-15 gas system consists of four varied lengths:
- Pistol-length (4")
- Carbine-length (7")
- Mid-length (9")
- Rifle-length (12")
Picking the right gas system is surprisingly easy: If you're building a typical 16" rifle, a mid-length system is the optimal choice. It provides the most reliability and comfort by reducing recoil compared to a carbine-length system. If you're building an SBR or pistol with a 14.5" to 11" barrel, stick with a carbine gas system. If you're going below 11" in barrel length you'll need a pistol-length gas system.
Pictured above is the standard low-profile gas block. If you wish to utilize a two-piece handguard that is not free-float, you'll likely instead need to invest in a barrel or gas system that utilizes the A2-style front sight post found on service-issued M16s and M4s.
The Stripped Upper Receiver
The stripped upper receiver is the simplest component in the upper assembly. Its job is to house the bolt carrier group and barrel. The threads for seating the barrel and barrel nut are universal, specific to the caliber or type of AR you're building (5.56, 9mm, or .308).
Nearly all stripped uppers are made with forged or billet aluminum. Some niche manufacturers have developed polymer and carbon fiber uppers for ultralight builds, but they're typically more expensive and less durable. You can save a few bucks if you forego the forward assist button, which aids in forcing the bolt carrier into battery when the receiver is dirty, though virtually all receivers come with the forward assist by default.
Bolt Carrier and Charging Handle
The bolt carrier group performs a few important functions. It allows the firing pin to strike the cartridge in the chamber, it ejects spent shell casings, and it chambers new rounds. Bolt carrier groups are standardized like the stripped upper receiver: There are no differences in functionality between various brands and manufacturers.
Bolt carrier groups are profiled as "full-auto/M16" and "commercial" carriers. Full-auto carriers are heavier and dampen recoil slightly, while commercial carriers are lighter in weight and work well with short barrels, pistol builds, and subsonic cartridges like 300 Blackout. All carriers are generally made from the same steel and feature a black phosphate or nitride coating. Many carriers also incorporate chrome lining inside the carrier and gas key to aid with cleaning and to prolong life expectancy.
Charging handles are standardized, too. You can grab custom handles with larger hooks for your fingers, though they all perform the same function. Virtually all handles are made from aluminum.
Lower Receiver Parts
Stripped Lower Receiver
The only part of the AR-15 considered the firearm is the stripped lower receiver. Like we mentioned above, you can pick a stripped lower from an FFL and conduct a background check, or you can fabricate a receiver using an 80% lower. Both units are available in forged and billet aluminum. Forged receivers are considered "mil-spec" while billet receivers typically include upgrades, like an integrated trigger guard and threaded fittings for the lower parts kit. Billet receivers cost, on average, 10% to 20% more than a forged unit. You can also use a polymer stripped receiver or 80% lower if you decide to configure a lightweight build. Machining a polymer lower is easier than cutting and drilling an aluminum lower.
Both "retail" and 80% receivers have the same physical dimensions and use the same components for the same caliber. For 5.56 NATO and 300 Blackout, that means either option will use the same trigger, parts kit, buffer tube, and pistol grip.
Lower Parts Kit
The lower parts kit is the most complex part of the AR-15, but even its function and components are relatively straightforward. The LPK includes the trigger, hammer, operating springs, grip, safety lever, magazine catch and release, and bolt catch and release. Installing the LPK requires just a few common tools like a punch, a small blade, and a small gunsmithing or mini model hammer.
Parts kits don't vary in their design or construction unless you're investing in an aftermarket trigger, such as a drop-in match trigger or binary trigger. These custom triggers provide a lighter pull, or they provide a unique double-tap fire mode that is considered semiautomatic. In the early days of AR building, it was possible to install an M16 parts kit. Today, lower receivers and 80% lowers are manufactured in such a way that they cannot accept "full-auto" parts, like an auto sear, three-position safety lever, and the necessary third pin hole.
If you've already graduated to assembling your AR, here's a step-by-step picture guide that provides everything you need to know to install an LPK into your lower receiver.
The buffer tube acts as a housing for the recoil spring. When the AR-15 fires, the gas system forces the bolt carrier group into the tube via the gas key. The recoil spring and recoil buffer inside compress, then force the BCG back into battery. This chambers another round.
Virtually all standard AR-15 rifles come equipped with the buffer tube shown above. This "mil-spec" carbine tube is compatible with all standard AR-15 lower receivers that are also mil-spec. Mil-spec components are always forged, while commercial components are made from billet. This manufacturing difference means the threads on both tubes and receivers are different and incompatible with one another:
Regardless of which tube and receiver combination you go with, either setup uses the same recoil spring and buffer. If you're building a retro-inspired AR-15 with an original fixed stock (like an M16), then you'll need to invest in a rifle-length buffer tube, buffer, and recoil spring.
Recoil Spring and Buffer
There are various buffer weights that are designed to lessen recoil, extend bolt and receiver life, or work with unique cartridges like 300 Blackout or 9mm. For the 5.56 NATO-chambered 16" rifle, there are four common buffers to choose from:
- Carbine (3.0 ounces)
- Heavy, or "H" (3.8 ounces)
- H2 buffer (4.6-4.7 ounces)
- H3 buffer (5.0-5.4 ounces)
- Pistol buffer (5-8.5 ounces)
- Rifle buffer (5.0 ounces)
The general rule is this: You should run the heaviest buffer that will reliably cycle your bolt with any ammunition. For a 16"-barreled rifle with a carbine gas system, that typically means an "H" or "H2" buffer. For a mid-length gas system, that usually means an "H" or Carbine buffer. Pistol buffers are used for blowback-operated AR's, like those chambered in 9mm. The heavier buffer accommodates the pistol cartridge's added energy.
Virtually all AR-15s come with a Carbine buffer to guarantee reliability in any conditions. This lighter buffer typically makes felt recoil just slightly more noticeable. For most AR's in all calibers, the recoil spring is universal.