The AR-15, or Armalite Rifle 15, is America’s most iconic weapon. Originally developed in the 1950s to serve the U.S. military, the rifle was later adapted to become the M16, which is still in use by the armed forces today. The M16 was further developed into the M4A1 carbine. It all stems back to the AR-15.

More than 5 million Americans own an AR-15, many of them choosing to build their own customized model.

Why build an AR-15 rifle?

Building an AR-15 rifle is a great pursuit for any arms enthusiast. The process isn’t easy and you’ll have to climb some fairly steep learning curves along the way. By the time you’ve completed your rifle, you’ll have an intricate knowledge of how it works and a deep connection with it.

Building your own AR-15 is a fun and rewarding process that we strongly recommend. In the end, your rifle won’t be just some stock model off the factory line, it’ll be your rifle, built with custom parts to meet your specific preferences. For the advanced shooter, this is essential.

If you just want to get shooting now, check out our list of the best complete AR-15s and and the best AR-15s under $1000.

Decide on the main use case for your AR-15 rifle

Are you a hunter or a competitive shooter? Do you want an AR-15 for entertainment and hobby, or is this primarily a weapon for home defense? The first thing to consider when building your own AR-15 is why you want a rifle in the first place.

AR-15 rifle or pistol?

An AR-15 pistol caliber carbine (PCC), otherwise known as an AR-9 when chambered in 9mm, has a shorter barrel and shoots pistol ammunition. It’s better suited for home defense and can be shot in most indoor ranges, as well as 9mm shooting competitions. A PCC is not ideal for hunting and long-range shooting.

Here’s our list of the best 9mm AR-15 uppers and the best complete AR pistols

Pros and cons of alternative calibers

Once you’ve decided on the type of AR-15 you want, the next step is to choose the right caliber. Each caliber has its own pros and cons. The standard caliber for an AR-15 is 5.56 NATO, although there are various alternatives.

.204 Ruger An all-round great caliber to shoot in many people swear by the .204 Ruger after they use it for the first time. Pros Low recoil is great for follow up shots

Shoots flat past 300 yards

Affordable Cons Not great for medium-sized game

Not ideal for hunting anything but varmints

Not great for home defense

.224 Valkyrie The .224 Valkyrie is another affordable caliber to shoot in that is better suited to hunting small game and long-range shooting. Pros Affordable caliber

Can use big-game bullets as well

Exceptional performance at long-range with minimal drop Cons Perhaps not ideal for some big game

Not great for home defense

.300 AAC Blackout Built to replace sub-machine guns with an M4, this cartridge will shoot supersonic and subsonic ammo with zero issues. Pros Great for short barrels

Shoots subsonic ammo with no issues

One of the best rounds for suppressed guns Cons Not suitable for shooting past 300 yards

Extremely loud without a suppressor

High penetration is bad for home defense

.458 SOCOM With insane stopping power, the .458 SOCOM is ideal for close-quarters hunting and a perfect fit for short barrel rifles. Pros Incredible stopping power

Works well with SBR and suppressors

Great for hunting out to 75 yards Cons Not ideal for long-range hunting

Not great for longer barrels

Expensive

.50 Beowulf This is a caliber that was initially intended to stop vehicles and car bombs, so you know that it’s going to pack a serious punch. Pros Incredible stopping power

Will put down anything that you could possibly aim your rifle at

Very fun to shoot Cons Eye-wateringly expensive

Lack of factory bullets means reloading is necessary

Kicks hard

6.5mm Grendel This was essentially designed as a round meant to mimic the 7.62x39mm rounds of the AK-47. It offers insane performance at long-range but is very expensive to shoot. Pros One of the best for hunting

Great for competitions

High ballistic coefficient and sectional density Cons Very expensive

Not necessary at close range

Burns out barrels

6.8 Remington SPC This round was developed for the M4 to shoot at long-range targets. The military declined to use it because most engagements take place at closer ranges. Pros Goes further and hits harder than 5.56

More penetration than 5.56 rounds

40% more energy than 5.56 rounds Cons Lack of magazines available

Doesn’t work with standard mil-spec mags

Expensive

Set a realistic budget for your AR-15, don’t skimp

One of the big advantages of building your own AR-15 is that you can generally do it for less than the price of a factory model. That being said, you should budget enough money to build the best AR-15 you possibly can, otherwise what’s the point?

How to select and purchase the best AR-15 parts

Choosing the best parts for your AR-15 isn’t going to be a quick process. You’ll need to do a lot of research on each component of the rifle and talk to other shooters who have built their own rifles. Forums are great for this, and there are loads of online resources available comparing parts.

A full list of AR-15 parts

An AR-15 consists of two main parts, the upper receiver, and the lower receiver. You can buy both in various stages of construction, from complete to 80% finished, meaning you’ll need to do some of the milling and drilling yourself. Stripped uppers and lowers let you pick all the components yourself.

Upper receiver Stripped upper receiver Barrel Handguard Charging handle Bolt carrier group Gas tube Gas block Upper parts kit

Lower receiver Stripped lower receiver/80% lower Fire control group (trigger) Safety Mag release Buffer Buffer tube Buffer spring Buttstock Lower parts kit



Barrels

The barrel is the most important part of your AR-15. It determines which caliber you’re shooting in, as well as how far you can shoot and how accurate your shots will be. There are a few things to consider when choosing a barrel.

Barrel length

Your barrel length will determine the velocity of your rounds. Shorter barrels are better for maneuverability and large muzzle devices, while longer barrels offer more velocity, and therefore more long-range accuracy when fired. In general, 16-inches is the Goldilocks length.

Barrel materials

When it comes to choosing a rifle barrel, your best bet is to go for the more affordable 4140 Chromoly, the more accurate 416R Stainless Steel, or the most popular choice, 4150 Chromoly Vanadium (CMV). These materials come closest to meeting or surpassing mil-spec requirements.

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Manufacturing and rifling methods

Rifling means twisted ridges cut into the bore that forces the bullet to spin as it’s fired, stabilizing its flight for accuracy. There are several methods of adding rifling to a barrel to consider when building your AR-15.

Cut rifling: This is the oldest method. It affords lots of customization and reduces stress on the barrel. It’s a slow process but it produces a solid result.

This is the oldest method. It affords lots of customization and reduces stress on the barrel. It’s a slow process but it produces a solid result. Button rifling: This is a process that gives more accuracy and consistent results. A ‘button’ is pulled through the bore as it turns, imprinting the rifling into the steel.

This is a process that gives more accuracy and consistent results. A ‘button’ is pulled through the bore as it turns, imprinting the rifling into the steel. Broach rifling: This is another modern method that is great for consistency but isn’t suitable for match-grade barrels.

Chrome lining

Getting a chrome-lined bore may reduce accuracy a little but greatly improves barrel life and performance when rapid firing. You can also choose melonite for better accuracy.

Twist rates

AR-15 twist rates are between 1:7 to 1:12. This refers to how many times the rifling does a complete rotation in one inch. For standard 5.56 or .223 Remington rounds, you want to stick with 1:7 or 1:8, probably the latter for practical civilian rounds.

Upper receivers and parts kits

Different uppers give your AR-15 a unique look and feel, although they generally all accept any barrel, gas system, and BCG unless you’re chambering your AR-15 in a special caliber like 9mm or .300 BLK. There are a few styles to consider, each with varying practical benefits.

A1/A2/A3/A4 uppers

The A1 to A4 uppers refer to the designs of the upper receiver from the original A1 upper from the 1950s AR-15 to the modern A4 upper that is found on most AR-15s now sold out of the factory.

The main thing to know is that A2 uppers have a carry handle, which limits the type of scopes and optics you can mount, while the A4 upper comes with a flat top rail that supports a variety of mounts.

Here are some of the best quick detach AR-15 scope mounts, and 1-4x scopes that are great for the AR-15 platform.

M4 uppers

The M4 flattop upper is essentially the same as the A4 upper. It has a Picatinny rail that supports pretty much every optic and accessory on the market. This makes it the most popular choice for most factory and custom AR-15s.

Complete vs stripped uppers

Complete upper assemblies are pre-assembled and include the barrel, gas system, handguard, and so on. They’re drop-in systems that are much easier to install, making them ideal for novices who just want to get on with the shooting.

Stripped uppers come with just the upper receiver itself, so you can customize the upper assembly any way you like using individual parts of your choosing.

Our picks for the best AR-15 uppers

Bolt carrier groups (BCGs)

The bolt carrier group is responsible for firing, ejecting, and loading rounds. BCGs are designated into semi-automatic and fully-automatic designs. Fully-automatic bolt carriers do not make your AR-15 fully auto. However, full-auto BCGs are generally more reliable than semi-auto BCGs. You have various materials and finishes to choose from, a good choice is chrome-lined Carpenter steel.

Charging handle

Most charging handles do the job the same way. The most you can really expect is for one to have a bigger grip than the other, or to be better suited to lefties.

We’ve found the best AR-15 charging handles out there.

Dust cover

Dust covers for the ejection port are also pretty much standard, although you might want to get something printed on yours to give your AR-15 that extra touch of spice. It is possible to go without a dust cover entirely if you prefer.

Handguards

There are various styles of handguards that change the aesthetic of your AR-15 and allow for different mounting systems. The most important thing is that the length isn’t too short for your gas system.

Check which barrel and gas system length the handguard is designed for before buying it. Free float handguards can be longer than the gas block, while drop-in ones will need to be the exact length to attach onto your gas block. Free float handguards are more accurate, more expensive, harder-to-install, and have better accessory mounting options than drop-ins.

Here are our reviews of the best AR-15 handgurads.

Gas system parts

The gas system on your AR-15 uses hot gas from each shot fired to force the BCG back, eject the spent cartridge, and chamber a new round. This is what makes an AR-15 a semi-auto weapon. There are two main gas systems to choose from.

Direct impingement vs piston gas systems

Both of these systems do exactly the same thing, but the direct impingement method forces the dirty gas back into the receiver, whereas the piston gas system ejects the gas outward so it doesn’t eventually foul up the firing mechanism.

A direct impingement system is the military standard, but will require more cleaning.

Gas block options

The gas block is the valve that controls the gas going up the tube. When it comes to gas blocks, you have two options: adjustable and non-adjustable. Adjustable gas blocks allow you to tune the amount of gas that gets let up the gas tube and back into the receiver.

A non-adjustable gas block can’t be tuned and is generally considered safer for beginners as you won’t have to worry about tuning your rifle incorrectly. Default setups usually let more gas through than is necessary and this can lead to wear over time.

Another choice is between low-profile gas blocks or front sight blocks. The latter is the classic A-frame option with the front iron sight on top. These work best with drop-in handguards.

Gas system lengths

Generally speaking, a mid-length or carbine gas system will fit most AR-15s. Pistol and rifle gas systems are designed for custom builds using special ammo like .300 Blackout. If in doubt, go mid-length.

Gas system Barrel length Gas system length Pistol Less than 10″ 4″ Carbine 10″ to 18″ 7″ Mid-Length 14″ to 20″ 9″ Rifle More than 20″ 12″

Lower receivers and parts kits

The lower receiver is one of the most important sections of your AR-15 and you’ll need the following components, which are often available as part of a kit:

Grip

Safety

Trigger

Hammer

Bolt catch

Buffer retainer

Magazine catch

Takedown pins

Check out our picks for the best AR-15 lowers and the best lower parts kits.

What is an 80% lower and how to complete it

An 80% lower is a lower receiver that hasn’t been fully machined to completion. The lower is the only part of an AR-15 that is legally registerable as a firearm. Because it’s unfinished, an 80% is not yet legally registerable. That allows the buyer to get around extra taxes, fees, and waiting periods when buying it. To complete an 80% lower, you’ll need to mill out the fire control group. You can do this yourself or give it to a gunsmith.

Here’s our list of the best 80% lower receivers

Complete lowers vs stripped lowers vs 80% lowers

Complete lowers are pre-assembled with the trigger assembly, safety, and mag holder in place. A complete lower can also have a buttstock and buffer tube. A stripped lower is just the fully-machined lower receiver without the extra parts added on. Both complete and stripped lowers are serialized and are thus considered firearms.

An 80% lower isn’t considered a firearm and you could buy one now and have it delivered tomorrow without any hassle. Obviously, complete lowers are more convenient for beginners, though 80% lowers will be best for those who want to avoid taxes or BS paperwork.

Furniture: stocks and pistol grips

These accessories can make your rifle look cool and ensure that it’s comfortable for you to shoot. A good quality AR-15 stock is essential for comfort and stability when shooting. The furniture includes the parts that make the most contact with you, and which transfer any recoil to your shoulder, especially if using an alternative caliber, so make sure your furniture suits you perfectly.

You can also add a brace to your AR-15, which attaches to your arm and improves stability. There are many options when it comes to dressing up your AR-15, we recommend that you physically try some out in a store or at the range so you can determine which is most comfortable for you. Standard stocks are fortunately adjustable.

Pistol grips can be customized in various ways, for example, to have finger dips or to be flat, depending on your preference. Here’s our list of the best AR-15 pistol grips. You can also consider getting an AR-15 pistol brace.

Buffer tube parts

The buffer, buffer tube and spring must be balanced to the amount of force transfered from the gas system. A 16″ barrel chambered in 5.56/.223 will usually require a standard (or ‘carbine’) buffer tube. It’s 7-inches long and weighs exactly 3 pounds. You’ll probably want a standard buffer spring.

Fire-control groups (triggers)

The right trigger will improve your accuracy and control. Many shooters say that the trigger is the most important thing that you can upgrade on your weapon.

Single-stage vs dual-stage

Single-stage triggers are simple: you pull back and the gun goes bang. This makes them ideal for quick repeating shots.

Dual-stage triggers have an initial stage where you pull back some of the slack and then a second, usually much lighter trigger pull, that fires the round. This means you can fire without the weight of the trigger pull throwing your aim off.

Here’s our list of the best AR-15 dual-stage triggers

Drop-in triggers

These are pre-assembled trigger assemblies that you can quickly install into your AR-15 without much hassle. They’re assembled to the highest standards, offering no real down-sides.

Adjustable triggers

These triggers allow you to change the pull weight of the trigger. This means that you can have a lighter weight for long-distance target shooting, and a heavier weight for, say, home defense when a slip of the finger could be fatal.

Sighting systems

Your AR-15’s sights will play a huge role in how accurate you can shoot at longer distances. There are many types of scopes and optics on the market, each with their own benefits and drawbacks. What you will use your rifle for will determine which sighting system you’ll want.

Iron sights

They might seem a little unadventurous, but many people still value the classic iron sights. There is nothing more reliable than iron sights. If you want your AR-15 to look like the classic version or the M16 rifles used in the Vietnam War, then having an A2 upper with a carry handle will give you the best run at it.

Iron sights never break or run out of batteries

They can be very accurate at closer-range targets

They look classic

They’re cheap

Check out the best AR-15 iron sights here.

Scopes

When you’re shooting long-distance targets and hunting, a scope will be what the doctor ordered. Scopes for AR-15 rifles come in all kinds of magnification levels, from 4x to 20x and everything in-between. Scopes have different reticles, some of which will help you to judge a bullet’s windage and drop at different ranges.

Essential for long-range

Come in many levels of magnification

Be careful about the weight of the scope

They can get very expensive

Check out our reviews of the best scopes for your AR-15. Click here for night vision, here for the best long-range scopes, and here for the best from legendary brand Leupold.

Red dots

The classic red dot sight reflects an LED beam of light onto the front glass of the optic and back into your eye. It’s excellent for mid-range shooting. Although magnification is an option, the dot may then take up too much space on your target.

Ideal for close-quarters shooting

Quick target acquisition

Cheap

Holographic sights

Holographic sights are similar to red dot sights, but they use different technology and tend to have a circle around the dot in the reticle. The laser hologram technology behind holographic sights means that the reticle is seemingly placed in front of the sights and superimposed onto the target.

This makes it quick and easy to acquire targets. The reticle also stays the same size when magnified, meaning these sights are better suited to hunting and shooting at a distance. You can smash out the front glass on the optic and the holographic sight will still work.

Great for mid-range shooting and hunting

Still very effective at close range

Works even when the glass is broken

Expensive

We’ve reviewed the best holographic sights for your AR-15

Muzzle devices: muzzle breaks, flash hiders, or compensators

There are several tools that you can attach to your muzzle, here are the main ones to consider.

Muzzle breaks: These force gas to escape through the sides and back of the muzzle and break, instead of just the front, which reduces recoil. Here are our picks of the best muzzle breaks out there.

These force gas to escape through the sides and back of the muzzle and break, instead of just the front, which reduces recoil. Here are our picks of the best muzzle breaks out there. Compensators: These clever little things work similarly to a muzzle break, only the vents for gas to escape are most prominently located on top of the compensator. This stops your muzzle from rising up when firing, thereby improving the accuracy of follow-up shots.

These clever little things work similarly to a muzzle break, only the vents for gas to escape are most prominently located on top of the compensator. This stops your muzzle from rising up when firing, thereby improving the accuracy of follow-up shots. Flash hider: Any guesses on this one? That’s right, flash hiders greatly reduce the flash from your muzzle, meaning you can conceal your position when shooting.

Learning the AR-15 lingo

If you’re new to this game, then you’re no doubt finding yourself dumbfounded at some of the words shooters are throwing out there. Here’s your guide to some of the AR-15 lingo.

M4 feed ramps

M4 feed ramps are cuts in the barrel extension and upper receiver that make it easier to feed rounds from the magazine. They come in two main types, ‘rifle’ and ‘M4.’ Just make sure that you never use a barrel extension cut with rifle feed ramps with a receiver that has M4 feed ramps.

Low-profile gas blocks vs front sight bases

Whether you choose a low-profile gas block or a front sight base will depend on what handguard you want to use. Classically styled AR-15s feature front sight bases clamped to the barrel, while shooters who install a free float handguard will opt for a low-profile gas block.

Top rail T-marks

Top rail T-marks are the markings on top of a Picatinny rail. These markings make it easy to remember exactly where to re-attach an optic or other accessory in case you remove it. Whether you want this or not really comes down to personal preference.

M-LOK

M-LOK stands for Modular Lock and it’s an alternative rail system that is popular thanks to its cheaper cost and compatibility with plastic handguards. It’s lighter than the classic Picatinny rail system and more widespread than the KeyMod system.

If you’re wondering what to mount on your rail, check out our list of the bets AR-15 flashlights

There is a whole world of AR-15 components out there. Once you’ve built your rifle, check out the best AR-15 accessories to add to it.

Rifle length, mid-length, and carbine length gas systems and barrels

You’ll notice people talking about whether it’s best to have a mid-length, carbine, or rifle length barrel and gas system. Most people opt for mid-length or carbine. You should stick to matching the gas system length to your barrel to ensure the best performance.

Gas system Barrel length Gas system length Carbine 10″ to 18″ 7″ Mid-Length 14″ to 20″ 9″ Rifle More than 20″ 12″

Your work space

Once you have all your parts together, you’re going to need somewhere to assemble them. The kitchen table isn’t going to be ideal, let’s face it. You’ll need somewhere where you can spread all the parts out and not have to waste time packing them up at the end of the day. A distraction-free workspace with a workbench is essential.

You can easily build an AR-15 without any assistance from a gunsmith, but you’ll need to have the right tools at hand.

Roll pin punch set

You’ll need to use roll pins in your lower assembly, and this will require a roll pin punch. Trying to secure these pins in place without this tool is only going to end in tears.

Armorer’s wrench

An armorer’s wrench will be necessary if you’re installing your own barrel and handguard, buffer assembly, and more. It gets its name for good reason, so make sure you have one.

Action block

An action block clamps the lower receiver into a vice. You simply insert the action block into the lower receiver and then clamp it on.

Vices and clamps

You’ll need to have proper vices and clamps to secure the various parts of your AR-15 when working on them. Trying to put together your receivers without a vice and clamp is just a fool’s task.

Punches and stamps

You’ll need these to install various components during your AR-15 build.

Gun cleaning and maintenance mats

Building an AR-15 can be a messy process, and having a gun cleaning mat at hand will ensure that you don’t ruin your work surface with grease and oil, and it’ll prevent you from scratching your weapon on hard surfaces.

Gun cleaning utility brushes

You’ll need a proper gun cleaning utility brush to clean your AR-15 without scratching it. You’ll also need one of these to be able to reach the grease in all the nooks and crannies.

Gunsmithing fixtures

In order to keep all your parts in place when you’re working on them, you may want to invest in a gunsmithing fixture. This helps to hold everything in place.

Hammers

No tool bench is complete without a trusty hammer.

Screwdriver sets

With so many components to put together, a reliable set of screwdrivers will be needed.

Shooting glasses

It’s important to wear shooting glasses even when you’re building your AR-15 to protect against solvents, debris and flying springs.

Wrenches

Finally, you’ll need an Allen wrench set to tighten all the nuts and bolts.

Steps to begin building an AR-15

Now that you’ve got all your parts together and you’ve established a workspace, it’s time to get on with building your AR-15.

Have an AR-15 schematic to hand

This will help you identify all the parts and names. Many gun cleaning mats come with an AR-15 schematic on them.

The prep

Spread your gun cleaning and maintenance mat on your workbench

Arrange all of the parts in their respective assemblies

Have all of your tools to hand

Wear your shooting or safety glasses (there is a risk of parts pinging across the room)

1) Install the forward assembly onto the upper receiver

You’ll need:

Forward Assist

Forward Assist Spring

Forward Assist (Roll) Pin

Have the upper receiver upside down so the Picatinny rail is against your mat. Slide the forward assist spring onto the forward assist Use your roll pin starter punch to start the forward assist pin into the bottom of the upper receiver. Insert the forward assist into the upper receiver, ensuring that the pawl is facing the middle of the receiver. Slide it in. Now while compressing the forward assist assembly, push the pin in until it’s flush with the upper receiver. Once this is done, you shouldn’t have to hold the forward assist in place. Now use a large brass punch to drive the pin to make certain that it’s fully secure. It should be barely sticking out at this point. Finally, test your forward assist assembly to ensure it can freely move in and out of the receiver when it’s compressed.

2) Install the ejection port cover assembly on the upper receiver

You’ll need:

Ejection Port Cover

Ejection Port Cover Pin

Ejection Port Cover Pin Snap Ring

Ejection Port Cover Spring

Place the ejection port cover pin snap ring in the groove on the end of the ejection port cover pin. Carefully hammer the ejection port cover pin snap ring the rest of the way onto the ejection port cover pin. Use downward strikes or it will go flying… Place the upper receiver on its side so the ejection port is facing you and the barrel threats are on the right. Put the pin into the upper receiver with the snap ring on the end away from the receiver – just enough so it can stay in place without you holding it. Now place the ejection port cover on the upper receiver in the open position, lining up the holes with those on the upper receiver. Thread the ejection port cover pin down the first tunnel in the ejection port cover, stopping just before the pin gets to the middle of the cover (you’ll need to put the spring here before continuing). Take the spring and hold it with the short part sticking out in your left hand and the long sticking out part in your right hand facing you. Now put tension on the long sticking out part to force it into a half rotation. Hold this tension. Now place the spring onto the open ejection port cover with the long part on the right side and still under tension. The long part of the spring will be next to the pin that is halfway pushed through. Keep hold of the spring with your left hand and use your right hand to push the pin all the way through until the snap ring stops you. When you take your hands off, the ejection port cover should snap shut. For the final test, put your finger in the upper receiver and push the ejection port cover, making sure that it snaps open.

3) Assemble the bolt carrier assembly

You’ll need:

Bolt

Bolt Carrier

Bolt Cam Pin

Firing Pin

Firing Pin Retaining Pin

Push the bolt all the way into the bolt carrier, ensuring that the hole in the middle of the bolt is aligned with those in under the bolt carrier key (the tubes where screws attach to the bolt carrier). Ensure the extractor on the bolt is on the left side when facing the bolt face. Put the bolt cam pin into the large hole in the bolt carrier under the bolt carrier key. Slide the firing pin into the bolt carrier towards the bolt. Insert the firing pin retaining pin into the bolt carrier with the round end of the pin all the way down into the large recessed side of the bolt carrier. You’ll need to get the pin through the hollow bolt carrier and into the hole at the end, then out the other side. It’ll likely take a while to do properly. Test the bolt to ensure that it moves freely in and out of the bolt carrier.

4) Assemble the charging handle assembly

You’ll need:

Charging Handle

Charging Handle Latch Spring

harging Handle Latch Roll Pin

Charging Handle Latch

Insert the spring into the charging handle. Use a roll pin starter punch to start the latch roll pin into the top of the charging handle, just enough so it stands on its own. Compress the latch into the charging handle to line up the holes, then drive the latch role pin until it’s flush.

5) Installing the barrel

You’ll need:

Upper receiver

Barrel

Vice

Make sure the bolt carrier assembly and charging handle assembly haven’t been attached to the upper receiver yet. Slide the action block insert into the back of the upper receiver to prevent the vice from crushing it. It should make the ejection port cover pop open, close it after. Place the receiver into the action block and close it. Put the action block into your vise and tighten it (not too tight). Slide the barrel into the threaded end of the upper receiver, lining the pin on the barrel up with the slot in the receiver. Slide it in until the pin hits the slot. Keep your setup in the vice for the next steps.

6) Install the outer receiver nut and inner barrel retainer but on the barrel threads

You’ll need:

Outer receiver nut

Inner barrel retainer nut

Free float tube handguard

7 flathead handguard screws

7 handguard plug screws

2 sling swivel studs

Note: the following relates to free float handguards. Yours may differ.

Use a steel brush to remove the coating on the barrel threads. Put some grease on the threads. Remove the outer receiver and inner barrel retainer nuts from the handguard and separate them. Screw the outer receiver nut onto the barrel threads until it hits the ejection port cover pin, then unscrew it one full rotation so the gas holes line up on the nut and the upper receiver. Take the inner barrel retainer nut, coat the threads in grease, and screw it tightly under the outer receiver nut while holding the latter in place to keep the gas holes aligned. Use a wrench to make sure the inner barrel retainer nut is snug. Put the torque wrench into the takedown wrench spot. Torque to 35lbs, then loosen. Do this twice, then torgue past 35lbs to get the gas holes lined up

Your upper assembly is now complete and can be attached to your lower assembly.

Steps to building an AR-15 (lower assembly)

The lower receiver isn’t too tricky to put together, so let’s get started on some of the easier parts and work our way through.

1) Assembling the magazine catch

You’ll need:

Magazine spring

Magazine release button

Magazine catch

Put the magazine release spring (the longest spring in your kit) into the hole next to the magazine. Put the magazine release button on top of the spring with the serrated side facing up. Now push in the button, flip the receiver over and start threading the magazine catch in. Make sure you keep pressure on the magazine release button while doing this. Keep threading until you can thread no more!

2) Assembling the trigger guard

You’ll need:

Trigger guard

Long roll pin

Depress the pin in the trigger guard and slip it into the receiver. Start the roll pin with a plastic hammer to avoid scratches.

Here are our picks for the top AR-15 trigger guards

3) Assembling the bolt catch

You’ll need:

Bolt catch spring and plunger

Bolt catch

Bolt catch roll pin

Put the spring on the plunger and insert it into the hole above the magazine latch. Put the bolt catch on top of the plunger with the hole aligned to the holes above where you put the spring. Depress the bolt catch and place the roll pin through the holes to lock it in.

4) Trigger assembly

You’ll need:

Trigger

Trigger Spring

Disconnector

Discontor Spring

Hammer and Trigger Pins

Hammer

Hammer Spring

Secure your lower onto a vice. Insert the large end of the disconnector spring into the hole on the top-rear of the trigger. Attach the disconnector into the grooves on the trigger, above the disconnector spring, with the hook facing forward. Connect the parts with a slave pin. Attach the trigger spring with the legs facing forward by looping it over the studs on the side. Take the hammer and attach the hammer spring by looping it over the studs on the side, with the closed end of the spring facing the rear. Place the trigger assembly into the lower so the holes line up and insert a pin to secure it from the left side of the receiver. Place the hammer into the receiver with the legs of the spring resting on the pin holding in the trigger. Drive the hammer pin into the top hole on the receiver to lock it in, the cock the hammer by pushing it down.

5) Installing the safety and grip

You’ll need:

Safety selector

Safety detent

Safety spring

Grip

Hex nut and washer

Cock the hammer to open the safety selector hole and insert the safety selector. Set the safety to fire. Put the safety detent in the small hole on the opposite side of the safety indicator. Put the safety detent spring in the small hole on the top of the grip so it sticks out. Now get the grip and bring it to the lower receiver so that the spring matches up with the safety selector hole – make sure the safety is still set to fire. Screw in the grip with the hex nut and washer.

6) Installing the takedown pin and buffer tube

You’ll need:

Rear takedown pin and detent

Detent spring

Buffer retainer and spring

Buffer tube

Castle nut

End plate

Find the shorter pin – the rear takedown pin and a detent. Install the rear takedown pin from left to right into the receiver just in front of the buffer. Now put the detent and in the small hole on the rear of the receiver, then insert the detent spring into the same hole. Now get the buffer retainer and spring and insert them into the hole in the buffer on the top of the receiver. Take the buffer tube and screw the big castle nut onto it, with the smaller square cutouts facing towards the front of the weapon. Now take the end plate and attach it to the buffer tube with the protrusions facing the back of the receiver. Make sure the alignment is correct. Now begin screwing the buffer tube into the receiver, being careful not to dislodge the detent spring. Stop screwing when the buffer tube reaches the buffer retainer. Now you’ll need to finish by screwing in the castle nut until the end plate is perfectly up against the receiver, holding in the spring. It should look like the picture below.

7) Final touches: front pivot pin

You’ll need:

Front pivot pin

Screw

Detent spring

Put the front detent spring into the hole at the front of the receiver. Now put the detent in after the spring and hold them in place. Get the front pivot pin and insert it through the two holes to keep everything in place.

8) Attaching the stock

You’ll need:

Stock

Buffer

Buffer spring

Simply slide the buffer spring down the buffer tube with the buffer end facing out. Then slide on the stock until you can’t easily move it anymore. Pull out the adjustment tab on the stock and choose your preferred distance, then push the tab in and bam! You’ve completed your lower receiver.

Now that you’ve built your AR-15, check out the best accessories to use with it!

FAQs on building an AR-15

Many questions will arise when you’re building your AR-15. Here are some of those we’re most commonly asked.

Q: Do I need to be licensed or a gunsmith to build an AR-15?

No, but some skills and tools help if you want to mill out an 80% lower.

Q: Is it legal to build an AR-15 at home?

Yes, it’s totally legal.

Q: Do I need an FFL or background check to build an AR-15?

Yes and no. You can do everything without an FFL or background check except purchase a lower receiver. You can buy an 80% lower to get around the checks.

Q: Does the AR-15 I build need a serial number?

No, unless you live in California, then yes. Always check the most recent state regulations.

Q: Do I need to register the AR-15 I build?

It depends on your state. Most of the time you won’t have to, but in New York you will. Check local state registry laws.

Q: What are the most common mistakes on first-time AR-15 builds?

Nuts and bolts are loose or over-torqued; wrong combinations of parts such as barrel and gas tube lengths; and mashed up springs.

Q: How many AR-15s can I build?

As many as you want.

Q: If I build an AR-15 for myself, can I sell it later?

The private sale laws cover selling home-built firearms. Make sure you check your state laws.

Q: Can I buy all the parts I need to build an AR-15 online?

Yes.

Q: What is an 80% lower and why do I need one?

The lower receiver is the only part that is legally considered a firearm. 80% lowers are incomplete, and thus they aren’t considered firearms and you won’t need any FFL or background checks to buy one online.

Q: What are the differences between forged, billet, and cast receivers?

Forged receivers have been hammered into shape while red hot, as have cast receivers. Billet receivers are made from a single block of metal and tend to be stronger, more aesthetically pleasing, and cost more.

You do not always have to go to a gun store to purchase an AR-15. Surprised? I know I was when I first ventured into the hobby of building custom AR-15s. Some of you may not know, but building your own AR-15 is actually quite simple. However, if you do not know anything about the process, or where to start, you have come to the right place. I am going to start this “How to Build an AR-15” series off with a list of frequently asked questions that I receive from individuals starting out with the hobby and possibly wanting to build their first AR-15.