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How to Build an AR-15: The Ultimate Guide for Beginners

How to Build an AR-15: The Ultimate Guide for Beginners

Posted by on Nov 9th 2022

If you're a first-time AR-15 builder, all the info found online can feel overwhelming. This simple guide introduces the basics to building your own. We'll gives you links to in-depth guides for each part of the AR, so you can become a master of the the rifle one step at a time.

What is The AR-15?

The AR-15 is a gas-operated, semiautomatic rifle developed by Eugene Stoner in the 1950s. Unlike civilian AR-15s, Stoner's design was fully automatic, given the designation "M16," and made to replace the then-current-issue M14 service rifle the U.S military. 

First issued in Vietnam as the M16A1, the rifle was continually updated and improved. Its civilian counterpart earned popularity in the consumer market over the following decades. 

Today, the AR-15 is the most popular rifle sold in the United States. The NRA estimates that well over 8,000,000 ARs (nicknamed "black rifles") were circulating in ownership in 2020. That number is estimated to be well over 20 million today. 

You're probably here because you want to build one! Smart choice. Let's begin.

AR-15 FAQs

Is the AR-15 illegal?

No. The 1994 Assault Weapons Ban temporarily restricted AR-15s, but it expired ten years later. Congress found no significant crime data supporting the law, as firearm crime rates generally remained the same during the ban. Some states (like California) have placed restrictions on the AR-15 to make its semiautomatic, 30-round-magazine configuration illegal.

Can I order an AR-15 online?

You can order any firearm online through a licensed dealer. If you're building an AR-15, you can order individual AR parts online without a dealer (Federal Firearms Licensee, or "FFL"). The only part of the rifle that's considered a firearm is the stripped lower receiver. To buy a stripped, serialized lower receiver, you need to go through an FFL.

You can also legally buy and fabricate your own stripped lower receiver using an 80% lower. Buying and fabricating an 80% lower does not require an FFL under federal law. State laws may vary. Check our "Shipping Restrictions" to see which states have banned 80% lowers.

What is an 80% lower?

An 80 percent lower an unfinished lower receiver designed for the AR-15. You can fabricate a stripped lower receiver using an 80% lower by cutting and drilling it yourself. Until you've started fabrication, the 80% lower is not legally considered a firearm by the Bureau of Alcohol, Tobacco, and Firearms (ATF) nor federal law. 

Using an 80% lower is an alternative to buying a stripped lower and paying the extra fees associated with buying a firearm through an FFL. Again, some states have banned these unfinished receivers. Check here to see which states have done so.

Read the full guide on 80% lowers here.

Is the AR-15 an "assault rifle"? 

No. An assault rifle is a fully automatic rifle (machine gun) intended for military and police issue. The AR-15 is a semiautomatic rifle. No matter how many politicians and news organizations claim otherwise, the AR-15 and all other semiautomatic firearms are not assault weapons by function nor legal definition.

Can the AR-15 be made fully automatic?

No. The AR-15's lower receiver is manufactured in such a way that an M16 or M4 (full-auto) parts kit, which is required for full-auto fire, cannot be installed without extensive modification. Making such modifications is illegal. Federal law prohibits the sale of machine guns to the general public, too. You need a special license to possess or make a machine gun.

What's the minimum legal barrel length for an AR-15?

Any rifle equipped with a buttstock must have a barrel measuring no less than 16" in length. A firearm with no buttstock may be considered a pistol. Only pistols may legally have barrels measuring less than 16" in length.

Any rifle equipped with a stock and barrel less than 16" is considered a short-barreled rifle, or "SBR." Owning or building an SBR requires submitting a special form to the ATF. The agency first conducts an enhanced background check before approving your request to buy or build an SBR.

Here's how to legally convert an AR-type pistol into an SBR.

What's the minimum legal rifle length?

Any rifle must measure at least 26" in overall length. That means if equipped with a 16" barrel, the rest of your rifle must measure at least 10" in length. If your rifle is equipped with a stock and measures less than 26", it will be considered an SBR. The ATF covers barrel lengths and firearm lengths in the  National Firearms Act Handbook.

What does "AR" stand for?

"AR" does not stand for "assault rifle". It stands for "ArmaLite Rifle", named after the company that sponsored Eugene Stone, inventor of the AR-15 and M16.

What are the AR-15's available calibers?

There are over 50 calibers available for the AR-15 platform. The most common calibers are 5.56 NATO, .223 Remington, .308 Winchester, 9mm, .22 LR., .243 Winchester, 6.5 Grendel, 6.5 Creedmoor, and 300 Blackout. 

Is the AR-15 a good hunting rifle?

Yes. Many hunters have traded traditional bolt guns for AR-15s. The plethora of calibers paired with semiautomatic fire, light weight, and high accuracy make the AR-15 the perfect hunting rifle. Chambered in 5.56/.223, the AR-15 can take smaller game like coyotes, hogs, goats, and small deer. Chambered in .308, the AR can take most North American big game.

What is the AR-15's effective range?

An AR-15 chambered in 5.56 or .223 and equipped with a 16" barrel has a maximum effective of 400 to 600 meters. AR-type rifles chambered in .308 Winchester or 6.5 Creedmoor maintain accuracy at 800 meters and beyond. ARs chambered in 300 Blackout or pistol cartridges (like 9mm) are accurate to between 100 and 300 meters.

Why Build an AR-15?


The AR-15 provides one of the lightest and smallest rifle platforms that's still capable of sending high-performance rifle cartridges (like 6.5mm loads) to distances of 600 meters and beyond. Its aluminum receivers and handguards are lightweight but incredibly tough.


Because the AR is so popular, endless amounts of barrels, receivers, parts kits, and accessories mean you can build an accurate, reliable AR-15 chambered in nearly any cartridge for $500 to $750. Its most common loads (5.56 and .223) remain the most affordable centerfire ammo today.


The saturated market means you can configure your AR-15 in hundreds of ways: Folding stocks, drop-in triggers, custom piston systems, carbon fiber barrels, lightweight custom receivers, new calibers for precision shooting -- the list goes on. You can anything from a 24"-barreled AR-15 that'll shoot a kilometer to a compact 9mm pistol for home defense, and just about everything in between.


The AR-15 is one of the most reliable rifles ever developed. Its simple and proven design means even entry-level rifles and parts are capable of firing thousands of rounds with good performance and few to no failures, with great accuracy. 

Building an AR-15 is easy

That's probably why you're here: Building an AR-15 requires just a few common tools and no professional gunsmithing knowledge. Let's talk about it.

How to Build an AR-15

 The AR-15 is separated into two halves: The upper receiver, and the lower receiver. Both components can be built piece by piece, or bought pre-built. Most first-time builders purchase a pre-built upper and put together a custom lower.

Upper Receiver Assembly

The upper receiver assembly includes: 

  • Stripped upper receiver
  • Barrel and chamber
  • Bolt carrier group
  • Charging handle
  • Front sight post
  • Gas system
  • Handguard

The upper assembly attaches to the lower assembly via two large "takedown" pins. 

Lower Receiver Assembly

The lower receiver assembly includes: 

  • Stripped lower receiver
  • Buttstock (rifle) or brace (pistol)
  • Trigger, hammer, and safety (lower parts kit)
  • Pistol grip
  • Buffer tube
  • Castle nut
  • Latch plate
  • Recoil spring 
  • Buffer

These technical guides cover each part of the AR-15 in detail:

These guides provide all instructions and tools needed to build an AR-15:

Now, let's go over building each half of the AR.

Building The Lower Assembly

To form a complete lower assembly that's ready to function, you'll need all the parts below.

1. The Stripped Lower Receiver

The stripped lower is the only component of the AR platform considered a firearm by law. All other parts of the AR-15, including the upper receiver, are not considered firearms. Those parts are bought and sold like any other non-firearm part or accessory online or in a store. 

You have two options to pick from when it comes to getting a lower for your build:

Option #1: Stripped Lower Receiver (firearm)

The stripped lower receiver is a serialized, ready-to-assemble firearm in the eyes of the law. It is not functional as a firearm by itself. It doesn't have a barrel, stock, grip, trigger, or upper receiver, and no other internal parts.

Even still, you must go through the same legal paperwork to purchase a stripped lower receiver as you would if you were buying a complete firearm. You must fill out and pass a background check (Form 4473), pay some fees to the FFL (Federal Firearm Licensee), and pay some taxes.

Option #2: 80% Lower Receiver (receiver blank)

Option 2 is why exists: Under federal law, you may instead create a stripped receiver at home to start your build by picking up and fabricating an 80% lower. This is a receiver blank that is not considered a firearm until fabrication begins, according to the ATF.

Look closely and you'll see the internal cavity for the trigger and parts kit is not machined. These units are also missing the holes for the hammer, trigger, and safety lever pins. You must drill these holes and cut the internal cavity yourself to create a stripped lower receiver. The Gun Control Act of 1968 says this is legal to do.

Build it: Buy an 80% lower here

Learn more: What's an 80% Lower?

Learn more: 80% Lowers & Frames Compared

2. Lower Parts Kit

The lower parts kit (LPK) controls the function of the hammer via the trigger's sear and disconnector, allowing for semiautomatic fire. It also allows the shooter to select "SAFE" or "FIRE" via the safety lever. Lastly, it's responsible for holding and releasing the magazine in the lower receiver. It includes a catch-and-release mechanism for the bolt, located in the upper receiver.

Build it: Buy a lower parts kit here.

Learn more: How to install the AR-15 lower parts kit.

Let's take a closer look at the lower parts kit's individual pieces.

Fire Control Group Components

The fire control group contains the most important pieces of the LPK: The hammer, hammer hook and hammer spring, trigger and trigger spring, disconnector and disconnector spring, and the trigger and hammer pins for installing those parts in the stripped lower. All these parts sit inside the main cavity of the lower receiver.

Hammer & Hammer Spring

The hammer strikes the firing pin inside the bolt carrier group, causing the chambered cartridge to ignite, sending a round through the barrel. The hammer spring provides tension on the hammer when it is cocked. When the hammer is released, that spring tension provides the force that is necessary to strike the firing pin hard enough to ignite the round's primer.

Trigger & Trigger Spring

The trigger releases the hammer when it is cocked. The trigger assembly includes the trigger spring (pictured above, wrapped around the trigger itself), the disconnector spring, and the sear. The sear is the small bar above and in front of the trigger bow. It is responsible for holding the hammer in the cocked position.

Once a round is ignited, its expelled gasses force the bolt carrier group to drive rearward. This action forces the hammer back down into the cocked position. Since the trigger is still depressed by the shooter's finger when this action occurs, the disconnector must temporarily hold the hammer in the cocked position until the trigger is released.

Disconnector & Disconnector Spring

The disconnector sits atop the trigger and attaches to the trigger via the trigger pin. As described above, the disconnector catches the hammer after the bolt rides backwards, holding the hammer down until the sear on the trigger engages the hook on the hammer. The disconnector spring provides the necessary tension for the disconnector to latch onto the hammer via the hammer hook.

Hammer Hook

The hook is located on the back of the hammer. The hammer hook interlocks with the raised, hooked portion of the disconnector. It is this temporary connection between the hook and disconnector that keeps the hammer held in the cocked position until the trigger is reset.

Hammer & Trigger Pins

The hammer and trigger pins are responsible for keeping the entire fire control group secured inside the lower receiver. To ensure the hammer and trigger springs remain seated (and to help prevent the pins from sliding out of their holes in the receiver), each pin has small notches for the legs of the hammer and trigger springs to be seated upon.

How the fire control group functions

To summarize all the actions above:

1. The trigger is pulled, releasing the hammer from the sear.

2. The hammer strikes the firing pin inside the bolt carrier group (BCG).

3. The chambered round ignites, and spent gas forces the BCG rearward.

4. The BCG's rearward travel pushes the hammer back down.

5. The trigger isn't reset yet, so the disconnector catches the hook on the hammer.

6. The shooter releases the trigger, resetting it.

7. The disconnector releases the hammer, and the trigger's sear catches the hammer to keep it cocked.

8. The bolt drives forward, a new round is chambered, and the AR is ready to fire again.

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 assembly:

Selector Lever (“Safety”)

The selector lever, commonly called the “safety”, sits flush against the left side of the lower receiver. When engaged, or "SAFE", the rounded portion of the lever inside the receiver prevents the trigger from being pulled. When disengaged, or set to "FIRE", the lever rotates to expose a flattened portion that allows 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

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 lower's magazine well. When the button is depressed from the right side of the receiver, the catch is released and the magazine falls. The pictured spring keeps tension against the catch, preventing the magazine from releasing until the mag button is 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 inside holes located at the front and rear of the lower receiver. These pins are held in place by springs and detents. Pulling on the rear takedown pin allows access inside the upper and lower receivers to clean both assemblies and to remove the bolt. Pulling both pins allows for complete separation of the upper and lower receivers.

Bolt Catch Assembly

The bolt catch grabs the bolt carrier group when the last round in a magazine is fired, preventing it from driving forward into an empty chamber. The shooter can also manually engage the bolt catch by depressing the catch while pulling on the charging handle. The button on the catch releases the bolt, driving it forward and chambering a round once a fresh magazine is loaded. A roll pin secures the catch to the lower receiver. A spring and buffer provide tension for the catch to properly actuate.

Buffer Retainer

The buffer retainer rests inside the lower receiver, inside the buffer tube housing's threads. Its job is to prevent the recoil spring and buffer from decompressing and flying out of the tube when the receivers are separated. The small nib atop the retainer rests against the face of the buffer. To remove the buffer, one can depress the retainer and its spring.

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 those parts from falling out when pulled or rotated.

3. Buffer Assembly

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

Build it: Buy a buffer assembly here.

Receiver Extension (Buffer Tube)

The receiver extension (called a buffer tube) 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 buffer tube and compresses the recoil spring via the buffer. Once compressed, the spring and buffer forces the bolt to drive forward, chambering another round.

Learn more: Commercial vs. mil-spec buffer tubes explained.

Recoil Spring

There are two types of recoil springs made for the AR platform. The less common spring is designed for the fixed A2-type stock with a hidden buffer tube, seen on old M16s and modern semiautomatic replicas. This longer spring measures 12". The common carbine spring, found in virtually all AR-15s today, measures 10" and is designed for the adjustable buttstock and exposed buffer tube.


The buffer is responsible for handling the recoil and blunt force created by the rearward 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.

Learn more: The guide to AR-15 buffers and weights.

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.


The buttstock provides comfort and recoil control for AR-type rifles. It allows you to obtain a good sight picture and enjoy stability with your rifle while you send rounds downrange. The AR-15 generally uses two configurations (fixed or adjustable stocks) with the latter being the most common by a wide margin. Shown above is how the castle nut and latch plate are installed on the buffer tube to secure the tube and stock to the lower receiver.

Pistol Brace

Build it: Buy a stock or brace here.

Some builders prefer shooting a compact, pistol-type AR with a barrel shorter than 16". Under normal circumstances, any firearm equipped with a buttstock must have a barrel measuring 16" or longer. Building any rifle with a barrel measuring less than 16" means you're constructing a short-barreled rifle (SBR), which is an NFA firearm.

Owning an NFA firearm requires obtaining an ATF-issued tax stamp after submitting to an enhanced background check. If you choose to build an AR pistol, you can only use a pistol brace or you must keep your buffer tube exposed with no brace and no stock installed.

Building The Upper Assembly

The AR's upper receiver assembly is the "business end" of things: It's where the bolt chambers a round and the firing pin meets the primer, causing live rounds to exit the barrel. Like the lower receiver, the upper consists of systems of various parts that work together. These parts include the:

  1. Gas tube and gas block
  2. Stripped upper receiver
  3. Bolt carrier group
  4. Charging handle
  5. Handguard (includes barrel nut)
  6. Barrel

Build it: Buy a pre-built upper receiver assembly here.

Want to build your upper piece by piece? Links to parts are below.

Let's break each set of parts down and explain how they work.

1. The Gas System

The AR-15's gas system is a direct-impingement system: Gas expelled by the cartridge travels down the barrel, behind the bullet. A small hole in the barrel diverts some gas back into the upper receiver via the gas block and gas tube. This redirected gas is forced into the bolt carrier group, driving it rearward into the buffer tube. 

This expels the spent casing and allows the bolt to cycle, drawing a new round from the magazine and chambering it as it slams back into battery.

Gas Block

The gas block is fitted directly to the barrel. A small hole drilled in the barrel, called a gas port, allows gas to travel through the block into the tube. Most gas ports have a diameter of 0.063" to 0.089". Gas blocks are commonly found in two configurations, shown above: The classic "A2-style", which combines a front sight post and gas block into one unit (pictured left), and the modern, lightweight low-profile block, pictured right.

Gas Tube

The tube directs gas from the block to another port in the front of the upper receiver. The receiver's gas port is located directly above the barrel nut. Part of the tube enters the port and interfaces with the gas key atop the bolt carrier group (shown below). The length of the gas tube is dictated by selecting one of four available gas systems:

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

Build it: Buy a gas block and tube here.

Bolt Carrier Group 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.

Learn more: The 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 and ignited. It contains the bolt carrier group and charging handle, positioning the bolt so the hammer can release and strike the firing pin when the trigger is pulled.

The stripped upper is mechanically simple. It incorporates a dust cover and, often, a forward assist. Both features are optional and aren't required for the AR to function. The Picatinny rail atop the receiver is the most common method for installing rear iron sights and an optic.

Build it: Buy a stripped upper receiver here.

Build it: Buy rear iron sights or an optic here.

Build it: Buy a charging handle here.

Learn more: How to assemble the AR-15 barrel & upper receiver.

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.

Build it: Buy a bolt carrier group here.

Learn more: The guide to the AR-15 bolt carrier group.

4. The Handguard

The AR-15's handguard serves two purposes: Protect the gas block and tube from damage, and protect your hand from the heat generated by the barrel when firing. Handguards are configured as two-piece units, like the "old-school" plastic M4-type guard shown left, and one-piece handguards milled from aluminum, shown right.

Unless you're interested in building a clone or replica of an older M16 or M4, you'll probably want to invest in a one-piece handguard. They're lighter, they incorporate modern accessory attachment systems like M-LOK, KeyMod, and Picatinny rails, and they "float" over the barrel. That means they do not secure to the barrel via the gas block, which helps to improve accuracy.

Build it: Buy a handguard w/ install hardware here.

Learn more: M-LOK and KeyMod handguards compared.

5. The Barrel

The barrel is responsible for sending rounds downrange. The barrel exampled above is typical for any AR-15 chambered in 5.56 NATO or .223 Remington: It measures 16" in length, it's made from 4150 Chromoly Vanadium steel, and it features a 1:7 twist rate. It's drilled for a carbine-length gas port and it's coated with a Melonite finish.

Build it: Buy a barrel here.

If all these specifications sound confusing, don't worry. Let's break each of them down.

AR-15 Barrel Specifications


Length dictates how much barrel you're working with. The longer the barrel, the higher the velocity your rounds will achieve, which translates into better accuracy and range. Most AR-15 barrels chambered in 5.56 or .223 measure 16", simply because anything longer provides little extra benefit.

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 every seven inches. For a 1:8 twist rate, it would rotate once every eight inches, and so on. Heavier cartridges need faster twist rates. Most 5.56/.223 rifles use a 1:7 or 1:8 twist, as do most AR-15s chambered in 300 Blackout. ARs chambered in .308 Win and 9mm work best with a 1:10 twist rate.

Barrel contour.

Contour describes the exterior shape of the barrel. The barrel shown above is forged in the common "M4" or "Government" profile. This profile is thin behind the gas port, but gets wider toward the receiver and muzzle. Custom barrels might have fluting or dimpling to reduce weight. Others barrels may sport a single, large diameter from end to end. These are usually called "bull" barrels or "heavy" barrels and they're designed to improve stiffness and accuracy while also handling high heat from rapid fire.

Gas block diameter.

Each barrel will have the gas block diameter described among its specifications. Most 5.56/.223 rifles are forged with a 0.750" gas block diameter.

Gas system length.

Like we covered earlier, the barrel's gas system will be described as pistol-, carbine-, mid-, or rifle-length. Pistol and carbine gas systems are most common for 5.56/.223 and .300 BLK builds, while 9mm ARs use no gas system. Most .308 ARs work best with a mid- or rifle-length system.

Feed ramps.

The feed ramps are located inside the barrel extension, and they're responsible for guiding rounds from the magazine into the chamber. High-end barrels use "M4" feed ramps, which are mil-spec ramps considered to be the most reliable for 5.56 NATO, 300 BLK, and .223 Remington.

Interior barrel lining.

Barrels can be lined with chrome or QPQ nitride, which greatly enhances rifling lifespan and the barrel's ability to withstand heat from rapid fire. Stainless barrels are not lined with any treatment or coating. Today, most AR-15 barrels are treated with nitride (also called Melonite). This treatment provides similar wear resistance as chrome lining, but it tends to provide better accuracy.

Exterior coating.

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

Common AR Configurations

Rifle in 5.56/.223 (16" Barrel)

This is the most popular AR-15 configuration. It's the standard setup you'll see on most kits and rifles.

Optimal 5.56/.223 Specs

Twist Rate: 1:7 to 1:9

The 1:7 twist rate is best for most 5.56 and .223 loads. It handles heavier bullets (77-grain) for hunting and precision shooting, but it stabilizes the common 55-grain and 62-grain bullets just as well. If you're sticking to shooting only those lighter bullets, a 1:8 twist or 1:9 twist is optimal.

Gas System: Carbine- or Mid-Length

The carbine gas system is the most common system found on 5.56/.223 ARs. It's the most reliable, providing guaranteed cycling of the bolt carrier group even when the rifle is dirty. Carbine systems are technically "over-gassed", so the recoil will be more noticeable than a mid-length gas system. The mid-length gas system is reliable, too, and reduces felt recoil.

Barrel Length: 16" 

A 16" barrel provides optimal velocity for 5.56 and .223 cartridges. Longer barrels provide negligible gains and add unnecessary weight.

Rifle in .308 (20" to 24" Barrel)

The AR-10 came before the AR-15. Eugene Stone converted the .308 rifle to fire 5.56 NATO. The modern .308 AR is based on a platform designed by DPMS Panther Arms, called the LR-308. It matches the AR-10's design. The only difference is found in how its receivers are shaped. This setup is popular among precision shooters and hunters.

Optimal .308 Specs

Twist Rate: 1:10

The 1:10 twist rate is the most popular for .308 ARs because it handles the widest variety of ammo (147-grain to 180-grain) with the most accuracy. Virtually all match-grade .308 ammo is 168-grain, and 1:10 does an excellent job of providing high stability for this round.

Gas System: Rifle-length

The extra powder and energy produced by the .308 cartridge requires a longer rifle-length gas system. Short gas systems (like mid-length) will be too over-gassed and could result in a critical failure of the rifle.

Barrel Length: 20" to 24"

The .308 cartridge requires a longer barrel to achieve optimal velocity. Most .308 rifles use a 20" to 24" barrel. A 24" barrel adds 80 extra feet per second of velocity compared to a 20" barrel. At 20", enough velocity is achieved to provide optimal performance.

AR Pistol in 9mm or 300 BLK

The 9mm and 300 BLK rounds burn powder quickly. They're intended for barrels measuring less than 16", so AR builders tend to favor these cartridges when building a pistol configuration.

Optimal 9mm Specs

Twist Rate: 1:10

This is the most effective twist rate for all 9mm loads. It'll stabilize and provide accuracy for loads weighing anywhere from 90 grains to 150 grains. 

Gas System: None

The 9mm cartridge uses blow-back energy to cycle the bolt. No gas system is required on a 9mm AR.

Barrel Length: 6" to 12" 

A 12" barrel provides the highest velocity with the shortest possible barrel length for 9mm. Anything longer provides negligible increases in velocity. Since 9mm was designed to burn its powder quickly and achieve velocity in shorter handgun barrels, a barrel as short as 6" provides good accuracy at 150 meters, and is capable of keeping loads subsonic for suppressed fire.

We wrote an entire guide on the optimal AR-9 configuration.

Optimal 300 BLK Specs

Twist Rate: 1:7 or 1:8

The 300 Blackout cartridge is designed to fire as a lightweight supersonic and heavy subsonic round using a single configuration. To do this, a fast twist rate like 1:7 or 1:8 is required. These rates will stabilize the heavier sub loads without over-stabilizing supersonic loads. 

Gas System: Pistol-length

This is the only reliable gas system for 300 Blackout. These heavy rounds use small cartridges and much less powder compared to other .30-caliber rounds, so the shortest possible gas system is required to cycle the bolt with both loads.

Barrel Length: 7.5" to 10.5"

Like handgun cartridges, the 300 BLK cartridge burns its powder quickly. It's designed for a short barrel. Ballistic data says a 9" barrel is optimal, with any barrel longer than 10.5" providing only negligible gains. Barrels measuring 7.5" or 8" suffer negligible losses in velocity, making them ideal for a compact pistol build. 

Here's an in-depth guide on building the optimal 300 Blackout AR.

Federal vs. State Laws And Building an AR

Some states have banned AR-15 kits, certain AR-15 parts, 80% lowers, and certain other firearm parts, or have otherwise restricted the sale and ownership of products we sell. So, be sure to check your local and state laws. We can’t ship our products to certain states. Please check our Shipping & Return Policy before placing an order  (see restrictions).

That's all, folks! We hope our guide was helpful.

Still have questions about building an AR? Reach us here.

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, 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.

We are a national retailer of individual components and not all products depicted on this website are legal in every state. Shipping of various products found on this website are prohibited to some states (such as California, Connecticut, District of Columbia, Hawaii, New Jersey, New York, Rhode Island, and Washington). The information, pictures, text or products presented on this website are not a representation by us, and should not be understood by you, that any product or completed firearm is legal to assemble or own in your state of residence. We encourage each and every builder to perform their own research about the state and federal laws that apply to them. It is your responsibility to understand the law and we encourage you to consult with an attorney or your local ATF representative.