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How to Build an AR-15 Upper Receiver: A Visual Guide

How to Build an AR-15 Upper Receiver: A Visual Guide

Posted by on Mar 29th 2024

If you've found this guide, you already know what you need -- we've got you covered: Follow these detailed instructions to assemble your AR-15 upper receiver from scratch!

(These instructions are for all AR uppers)

You can follow these instructions to build a standard AR-15 upper chambered in any caliber. These instructions are also applicable for AR-10 and AR9 uppers.

Build an AR Upper: Parts Required

This guide covers all the install steps for transforming a stripped AR upper into a fully functional upper assembly that's ready to be pinned to a lower assembly (to complete your lower assembly, follow these instructions).

Before we begin, you'll need the following parts:

Stripped Upper Receiver

We're using a standard, M4-type AR-15 upper receiver. This has a flattop Picatinny rail, forward assist mount, and dust cover mount. 

Dust Cover, Forward Assist

Your stripped upper may or may not come with the forward assist and dust cover preinstalled. We're covering how to install these two components in this guide. Not all uppers have forward assists, nor dust covers -- if yours has neither, you can skip these steps.

We're installing a mil-spec dust cover in this guide. If you want to make the install of your cover easier, you can upgrade to one with a preinstalled spring and pin, like the UTG Quick Install Cover.

Bolt Carrier, Charging Handle

We're using a standard M16-type bolt carrier group, and mil-spec charging handle for this build.

Gas Tube, Gas Block

For this install, we're using an adjustable low-profile gas block. The adjustable gas block will allow us to dial in the rifle after assembly, ensuring the upper cycles reliably, without over-gassing and excess recoil.

Our barrel requires a pistol-length gas tube. Verify you have the correct tube length before installing the gas system. Your barrel's specifications will indicate what size gas tube is required.

If you're assembling a 9mm upper, skip the steps covering the gas system -- the 9mm AR doesn't use a gas block or gas tube.


For this guide, we're installing a 300 Blackout barrel. No matter what barrel and caliber you pick, the install steps you follow will be the same.


The handguard typically includes the barrel nut, which is required for installing the barrel. Before beginning the install, it's a good idea to check the gas block fits underneath the handguard you've selected.

We're using a Daniel Defense Mk18 RIS rail for this install.

Building an AR Upper: Tools Required

Here are the required tools you'll need for this install:

Vise: You'll need a secure vise to hold your receiver and barrel in place while you wrench on it.

Vise Block (or Reaction Rod): The block (or rod) allows you to clamp the receiver in the vise without damaging it. We're using a reaction rod.

Torque Wrench: A 1/2 torque wrench works best.

Barrel Nut Wrench: This special adapter is required for tightening the barrel nut.

Roll Pin Punches: You'll need to tape some roll pins into the gas block and tube to secure both.

Breaker Bar: You'll need a breaker bar to temporarily loosen the barrel nut. More on that later.

These are optional, but we strongly recommend you have these tools available to make this job easier:

Pliers: They're useful for holding the gas system's roll pins in place while you tap them in.

Barrel Nut Grease: You should grease the threads on the receiver and barrel nut. If you don't, they'll probably seize together permanently over time.

Allen Keys: Some gas blocks have set screws that help keep the block in place, in addition to the roll pin(s). Allen keys will be needed to tighten those screws.

How to Assemble The AR Upper

Let's get right to it! First, we'll install the forward assist, then the dust cover -- again, skip these steps if your assist and cover are already installed, or if your receiver doesn't have either.

Place the spring over the forward assist, then push the assist into its housing.

You'll need to tap the assist's roll pin into the receiver. It helps to get the pin started, then insert the assist. Then finish tapping the pin in, while gently pressing on the assist to align it with the pin.

Once the pin's tapped in all the way, you'll see it just recessed on the bottom of the forward assist housing. 

Now grab the dust cover and its components -- the rod, C clip, and spring. First, install the clip onto the cut-out in the rod. 

It helps to use pliers; gently squeeze the ring until it snaps into place. Then line up the dust cover with its mount on the receiver, and insert the dust cover rod half way.

Now comes the tricky part: You need to install the dust cover's spring. You'll need to align the spring with the cover's rod holes:

Now press and hold the left side of the spring with your index finger, and rotate the right side of the spring one full rotation, clockwise. Pliers are very helpful, here.

Once you've rotated the spring end, it should rest flat against the inside of the cover. Keep holding the spring in place, and insert the rod all the way.

Once the rod is fully seated, the spring should be captured.

Test that the dust cover functions by closing it and pressing it back open from inside the receiver. The spring should flip the cover down with a pop.

Next, let's install the barrel. 

Before you install the barrel, we again recommend you apply some grease to the barrel extension and threads -- this will prevent the barrel from seizing on the receiver over time. 

The small button on the barrel extension is meant to help align the barrel with the receiver.

Once the barrel's inserted, check inside the receiver and verify the feed ramps on the receiver align with the cut-outs in the star chamber. This ensures reliably feeding of ammo.

Next, let's install the barrel nut.

Remember to check the install instructions for your handguard and barrel nut before proceeding. 

Our Daniel Defense rail requires that we install a lock ring for the handguard before installing the barrel nut. Unless you're using the same rail, ignore this component for your install.

Make sure the crown on the barrel nut -- the indentations meant for the barrel nut wrench -- are facing the muzzle.

Refer to your handguard's spec sheet for the appropriate torque specs for the barrel nut. If no specifications are provided, default to 40 lb-ft. of torque.

When you tighten the barrel nut, ensure the wrench adapter and torque wrench are parallel; this ensures proper torque is measured.

You'll also need to loosen and re-tighten the barrel nut a total of three times to ensure proper torque is applied. Loosen the barrel nut with a breaker bar and the adapter. Don't use the torque wrench to loosen the nut; this may damage your wrench.

Once the barrel nut's torqued, it's time to install the gas block and tube.

Before installing any set screws or pins, test the fitment of the gas block and gas tube. Install your bolt carrier group, and use the gas key on the carrier to help align the gas tube.

Once the tube and block are aligned, tighten the block's set screws (if applicable).

After tightening the set screws, install the gas block's roll pin. Your pin may use one or two pins, depending on its design.

The roll pin(s) may be longer than the block itself. If that's the case, just make sure its centered inside the block.

With the gas block secure, you can hammer the gas tube roll pin into place.

The gas tube roll pin should sit flush inside the top of the block.

You've built an AR Upper!

Your AR-15 upper assembly is now technically complete -- we're not going over the handguard install, since each handguard's instructions are different. In most cases, you just need to slide the handguard over the gas block and tube, secure it atop the barrel nut, and tighten its screws to clamp it down.

You can now install your completed upper on your AR's lower. If your lower isn't assembled yet, follow our guide on how to put together your AR-15 lower receiver.

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