Home Gun Tips The Beginner’s Guide to Building an AR-15 Part Two: Assembly and Testing

The Beginner’s Guide to Building an AR-15 Part Two: Assembly and Testing

rifle held by man in the field

This article is the second part of a two-part series. Back in part one, I covered what you need to build your first AR-15 rifle and even gave some tips on what to choose.

Now, how about putting all of those parts together?

Building your own Armalite-style firearm doesn’t require any power tools. Though it’s better to use a shop, I’ve built one in my bedroom before.

Thighs make poor vises but are better than nothing!

If you built an assembled upper and an assembled lower receiver, building your gun is easy:

Put the two halves together and push in the pins.

Building is more complicated if you’re putting parts into a stripped receiver. Honestly, though, I’ve had more trouble assembling some LEGO models!


Assembling the AR-15 Lower

Let’s start with the lower receiver.

It’s the heart of the gun and is also the part with the serial number, so it’s what’s legally considered the firearm.

You can start building your lower in almost any order. The safety does need to go on after the FCG, though.


Fire Control Group

assembling Fire Control Group

Also called the trigger assembly, the FCG is the complex collection of parts to install—unless you have a drop-in FCG, which makes this one of the simplest steps.

Generally, though, you’ll have the trigger, trigger spring, hammer, hammer spring, disconnector, and disconnector spring.

The trigger spring goes onto the side posts. The squared-off loop needs to be under the trigger, in front of the shoe. The open ends of the spring turn at the end. Those need to face forward, not downward.

The disconnector spring fits vertically into the trigger with the wide end down.

The hammer spring goes around the hammer’s loops with the open end down and the loop up. Both ends enter the coil on the side closest toward the hammer’s face.

Place the disconnector into the trigger’s trench, aligning the holes. The disconnector spring should sit inside the square cutout in the disconnector.

Now, put those two parts into the lower, making sure the spring faces forward.

When the holes are aligned with the lower’s trigger pin hole, insert the trigger pin from the left. It has two notches; start with the unnotched side.

Then install the hammer by laying the spring legs on the trigger spring’s coils and lowering the hammer into the receiver, flat side forward.

Insert the hammer pin once the holes are aligned in same way as the trigger pin.


Bolt Catch

ar-15 Bolt Catch

You can mar your gun’s finish when installing the bolt catch, so you may want to cover the left side of the receiver with painter’s or electrical tape.

Use a roll pin punch to start the bolt catch roll pin from the back of the gun forward. It shouldn’t yet go into the open slot.

Put the bolt catch spring onto the plunges and drop it into the hole.

Then, hold the bolt catch with the paddle up and facing outward and put it into the slot. It can help to put a hole punch in from the side opposite the roll pin to align everything.

Then, depress the bolt catch slightly and drive the roll pin through. You can clamp it in with a special tool or squeeze it in with pliers.


Magazine Catch

ar-15 Magazine Catch

The magazine catch is easy to install. Start by inserting the magazine catch into the left side of the receiver, underneath the bolt catch.

Insert the magazine catch spring from the other side. It’ll go over the post.

Then, put the magazine catch button into its slot. Hold it in, which will push the magazine catch outward.

Spin the magazine catch clockwise to screw the post into the button until the post is near to the button’s face.


Trigger Guard Assembly

ar-15 Trigger Guard Assembly

Oddly enough, installing the trigger guard can cause the worst permanent damage to your lower receiver.

Standard trigger guard assemblies are just two parts. The trigger guard has a button and the roll pin causes the pain.

Depress that button and insert the trigger guard, with the button toward the magazine well. Make sure the guard isn’t upside down—you want as much space inside the trigger area as possible.

Align the trigger guard and receiver holes and start the roll pin. Make sure to support the ears. People have hammered them off!

Once the roll pin is started, I use a C-clamp to squeeze it the rest of the way in.


Pivot Pin

ar-15 Pivot Pin

The two holes in front of the magazine well are for the pivot pin.

Start by inserting one of two long, thin, identical springs. Atop that goes the metal detent.

Then, you need to depress that detent with a strong, thin object, such as a safety razor.

Insert the pivot pin from the right side with the trench facing the detent, which will hold it in place.


Receiver Extension and Takedown Pin

ar-15 Receiver Extension and Takedown Pin

The receiver extension (buffer tube) and takedown pin are installed alongside each other because the hold each other in place.

I’ve lost detents when swapping buffer tubes!

Put the takedown pin in the holes, from the right side, with the channel facing toward the rear of the receiver.

Drop the detent down the hole in the rear of the receiver and follow it with the takedown detent spring.

Screw the castle nut all the way onto the buffer tube (not tight!), with the large notches facing the rear. Then put on the end plate, with the projection facing the receiver.

Screw the buffer tube onto the receiver, but stop before it starts to cover the buffer retainer hole.

At that point, put the buffer retainer and spring together, and put them in the hole. Hold them down (I use a flathead screwdriver), and stop once the hole covers the buffer retainer’s widest part but not the pin. Make sure it’s up and down so the end plate’s bump fits inside the receiver.

That end plate holds the takedown detent spring in place.

Once everything is lined up, screw down the castle nut. Use your hands first then your armorer’s tool to tighten it further.

Many people recommend staking the castle nut so it doesn’t back off. But this is your first AR build, so you may want to upgrade those parts later.


Grip and Safety

ar-15 Grip and Safety

You’ll need to cock the hammer to insert the safety.

It goes in from the left side. Turn the selector to “Fire.”

Now, turn the receiver upside down and drop in the safety detent.

The safety detent spring goes in a hole in the top of your grip. Put your grip onto the receiver (this may be a tight fit), keeping that spring straight.

The grip is kept in place with a screw inside the grip.



rifle buffer and stock assembled

Put the buffer spring onto the buffer.

Depress the buffer retainer and push the buffer spring-first into the buffer tube. Use a flat section to slip it over the retainer. Then, when on the other side, rotate it so a rounded section is against the pin.



Slide on the stock (you may have to pull down on a sideways pin), and your lower is good to go!


Assembling the AR-15 Upper

The upper receiver has fewer parts than the lower. However, some of the parts have to be installed to proper tolerances to avoid ending up with a loose barrel!

Assembled receivers will come with a forward assist and ejection port cover. Stripped receivers don’t come with those, so let’s start with those assemblies.


Forward Assist

ar-15 Forward Assist

The forward assist (FA) is a button covered in a spring that catches serrations on the bolt carrier group to push them forward. You’ll likely have to slide on that spring.

The FA’s tip has a curved tooth. Angle that to face toward the receiver’s left, then put the forward assist into the hole.

Insert the roll pin from above, and hammer or clamp it through the FA’s hole until it’s flush with the top.


Ejection Port Cover

ar-15 Ejection Port Cover

Don’t wait to install the ejection port after you install the barrel. It’s much easier to install beforehand.

The ejection port hinge pin has a notch for the snap ring. Install it using a pair of pliers, but be careful—it likes to pretend to be installed, only to jump off and hide in the next room!

Position the ejection port cover between the holes under the ejection port with the cylinder upward, so it’ll be inside the receiver when the cover is closed. Lay it all the way open for now.

Insert the hinge pin from the forward hole.

Once it passes through the port cover’s first section, insert the sprint. The long end goes down, against the cover, and the short end goes down, against the receiver, just below the ejection port. You’ll have to twist it clockwise so it holds the ejection port open.

Then, push the hinge pin all the way through, until stopped by the snap ring. This is surprisingly fiddly!



ar-15 Barrel

Now that the receiver itself is assembled, let’s put on the barrel.

You want a vise for this step, preferably with a barrel block so you don’t crush the upper receiver.

The barrel goes in easy enough. Slide the barrel extension into the receiver. The barrel’s index pin goes inside a slot on the receiver. You may need to apply some oil for smooth insertion.

Now you need to put on the barrel nut. This screws over the receiver and holds the barrel in place. There are many different barrel nuts out there, but all of them screw on.

Apply a light layer of grease to the threads before you put on the barrel nut. Make sure no grease gets inside the receiver, though!

You can use an armorer’s wrench or handguard tool to tighten the barrel nut. That tool will have a square hole for a torque wrench. Set that to 35 ft-lb.

Tighten the barrel nut to 35 ft-lb. Loosen it. Tighten it again. Repeat again, if you want.

If using an old-fashioned barrel nut with cutouts for a gas tube, continue tightening until one of the holes matches the X-shaped hole in the upper receiver.


Gas Block and Tube

Gas Block and Tube

Next, you need to attach the gas tube to your gas block. One side has an open end, and the other has three holes. That’s the end which goes into the gas block.

Insert it with the forward, larger hole facing down, and align the holes. You’ll insert a roll pin through those holes, which is easy with a vise.

Slide the gas block onto the barrel and insert the gas tube into the upper receiver. The tube shouldn’t touch any side of that X-shaped hole.

The gas block will stop against the barrel’s shoulder. Some barrels are dimpled so you know where to put the set screws.

Otherwise, you’ll have to line the gas block’s gas hole with the barrel’s gas hole. You can do this by marking the barrel’s middle line with pencil, doing the same on the front of the gas block, and lining them up.



Your handguard will come with its own set of instructions. Each one is different.


Muzzle Device

ar-15 Muzzle Device

The muzzle device screws onto the end of the barrel. You’ll want a washer between the device and the barrel so you can time the muzzle device properly.

Crush washers are most common. Put the smaller side toward the barrel and torque the muzzle device until it’s perfectly flat.


BCG and Charging Handle

ar-15 BCG and Charging Handle

Your bolt carrier group should come fully assembled.

Put the charging handle into the receiver from the rear with the hollow side down. Stop partway through, then insert the BCG so its tube is inside the charging handle’s hollow section (there is a wide area inside the upper receiver).

Then, push the BCG all the way forward.


Final Assembly

AR-15 assembled on table

Apply a thin layer of gun oil to all moving surfaces.

Then, align the holes on the upper receiver with the holes on the lower receiver. Push the pivot pin in then the takedown pin.

Congratulations! You’ve finished your rifle!


Function Testing

Once you’ve assembled both the upper and lower receivers and have put them together, there are a few things you need to test out.



ar-15 charging handle

First of all, make sure the charging handle and BCG work properly.

Grasp the charging handle and pull it all the way to the rear. Let it go. The BCG should slam forward.

Do this a few times to make sure it doesn’t get hung up on anything.


Trigger Function

ar-15 Trigger Function

Next, we need to make sure the trigger works correctly.

Make sure the gun is empty or loaded with a snap cap!

With the selector at “Fire,” charge the weapon. Pull the trigger. The hammer should fall.

With the trigger still held to the rear, charge the rifle again. The hammer shouldn’t fall when the BCG returns forward.

Slowly release the trigger. You should feel a clunk as the disconnector catches the hammer, but the hammer shouldn’t fall.

Then, turn the selector to safe. Pull the trigger and make sure the hammer doesn’t fall.

If your gun fails any of these checks, then reassemble the FCG. If it continues to fail, the FCG may be out of spec and require replacement.


Magazine Function

Insert an empty magazine. It should get caught by the magazine catch and not fall out.

Press the magazine catch button, and the magazine should fall out of your gun.

You may need to tighten or loosen the magazine catch so it slides in easily and drops free.



ar-15 Headspace

Lastly, you should check to make sure the bolt and chamber are within spec. This requires a set of GO/NO GO gauges, which you can buy or rent from your local gunsmith.

The BCG should be able to fully close on a GO gauge. It should not be able to fully close on a NO GO gauge.

If it closes on a NO GO gauge, then you might want to retighten the barrel nut and try again. Any other failure requires replacing the bolt, barrel, or both!


Firing Test

Man with AR-15 practicing target shooting

We’re not ready to load up a 30-round magazine yet!

The next tests come at the gun range.

Load one cartridge into a magazine, load it into the gun, charge the rifle, then fire.

The BCG should lock back.

Load three cartridges, load the gun, and fire. Each trigger pull should fire one shot, and the BCG should lock back on the last shot.

After that, I’m satisfied that the gun is working fine, and it’s time for some fun shooting!



Congratulations on finishing your first homemade AR-15!

Make sure to add some sights before you get to the range. Enjoy shooting your new gun, and feel free to upgrade components!

Eventually, you may find that your spare parts drawer has enough parts for another gun. Then you build it, upgrade it, and the cycle continues…


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