How to Assemble an AR-15 Upper Receiver

How to Assemble an AR-15 Upper Receiver

Building an AR-15 lower receiver is a fairly straightforward project that many gun owners choose to take on. What about building your upper receiver, though?

While it is a little more involved, it’s actually fairly straightforward as well if you have a little bit of knowledge and the right tools. 

What do you need to know and what are the right tools? I’m so glad you asked.

Not so long ago, I was facing the daunting task of building an AR-15 upper receiver with very little knowledge on how to go about it, other than a general knowledge of the inner workings of the AR platform. 

Now, almost a decade later, I’ve built around 20 ARs. Some of those were my own, and some were for customers or gifts for friends.

In that time, I’ve learned the AR platform inside and out and I’ve found (through much trial and error) the best tools for building an AR-15. If you’re looking to build an AR upper receiver, either to complete a build or just as a way to use up some spare parts, this should cover everything you need to know, step-by-step. 

Disclaimer: While the AR-15 is about as simple to assemble as a tricky Lego set, there are some tools involved, and we are going to be building a device that uses controlled explosions to fire pieces of metal at about four times the speed of sound...so be careful. 

This information is provided for educational and entertainment purposes, and if you’re unsure of something, always consult a professional. 

Alright, with that all out of the way, let’s talk tools.

Tools you’ll need for your AR-15 build 

how to assemble an ar 15 upper

You don’t need to spend much to build an AR-15 at all, but you do need a few specific tools to build your upper. Some of these things are general shop tools, and some are AR-specific items, and I’ve provided links to good quality options for each.

General tools and supplies

These are the general tools you’ll need. You may already have many of these things lying around, or be able to borrow them from a friend.

Bench Vise ($30): Any machinist will tell you, holding your work-piece in place is half the battle. You’re going to be punching and tightening a good bit, so having a stable way to clamp your upper in place is vital. 

You don’t need anything super heavy-duty, so a Harbor Freight model is fine. You can also get by without one but you’re not gonna have a fun time (ask me how I know).

Brass Punches ($25): Brass punches are great for getting pins into and out of where you need them to be without messing up your finish.

Jeweler's/Gunsmith Mallet ($8): A simple mallet like this one allows you to hammer on punches without worry about damaging anything with a missed strike. 

Grease ($13): For attaching the barrel nut and a few other things where we want to prevent binding. I’ve linked some mil-spec stuff, but a white lithium grease or even a general purpose gun grease or machine oil will work in a pinch.

AR-15 tools

These are the tools specific to working on the AR-15 that you’ll need. If you’re going to be working on ARs a lot, I recommend you go ahead and invest in these. If you’re just looking to build one or two guns, maybe see if you can borrow them from a buddy.

AR-15 Upper Receiver Block ($60): This is the most expensive AR-specific tool on the list, but it’s pretty much necessary if you’re going to be building an AR upper. This allows you to support your upper in the vise without damaging anything, including your finish. 

You can ignore this and just clamp the upper with nylon vise jaws, but this can be a little risky.

AR-15 Armorer’s Tool ($30): Good to have around if you’re going to be working on an AR, and you may need it to install your handguard if you aren’t using something that has a provided wrench or proprietary tool of some kind.

No-Go Gauge ($30): Another tool that isn’t particularly necessary, these are gauges used to check the chamber of your rifle. 

This particular gauge shouldn’t quite fit. If it does, you have a problem. Fortunately, the AR-15 platform is self-headspacing and doesn’t need to be tuned, as you’ll see in a minute, which makes these guys nice to have, but not strictly a must-have.

AR-15 upper receiver build parts list

Briefly, I want to cover all of the parts you will need in order to build your own AR-15 upper. 

Upper Receiver ($150): This is the basis of your upper. Everything attaches to this, and then this attaches to your lower, so go with something that is high-quality, like the Aero. I suggest going with an assembled option unless you want a custom dust cover. 

Barrel ($175): The barrel is the heart and soul of your upper, and will have the biggest overall impact on performance. If you’re looking for a rifle that performs well, this is where you should spend your extra cash. Also, make sure you get one that matches the caliber you want and that has a barrel nut.

Bolt Carrier Group ($140): The BCG is what allows the bolt to reciprocate inside of your upper, which drives the feeding, firing, and extraction process of the rifle. I’ve chosen a lightweight one here to keep recoil down, but there are about a million different options out there.

upper reciever assembly

Charging Handle ($45): You can feel free to skimp here and get the basic version for, like, $10, but a nice one like this Bravo Company example will make a huge difference in the ergos of your rifle, and adds some nice functionality.

Gas Block ($60): Your gas block can have a big impact on performance, particularly with a suppressor, so unless you’re on a very strict budget, I highly recommend going with an adjustable model like this one. 

Gas Tube ($15): Gas from your barrel has to be bled off to push the BCG back to cycle the gun. This tube, though cheap, is vital. You don’t need to spend a lot on this, just make sure you have the right size for the barrel (carbine, mid, or rifle length).

Handguard ($190): This gives you something to grip, and is a big part of attaching lights, grips, front sights, PEQ boxes, and whatever else you want to mount to your gun. I’ve linked to a fancy expensive one, but you can get basic models for very cheap. 

You’ll also want to check and see if your manufacturer calls for a torque screwdriver for installation, and grab one of those if necessary.

Muzzle Device ($-$$$): This device can be as basic or as fancy as you want. You can get a high-end competition muzzle brake to eliminate much of your recoil, or slap a cheap flash hider on there and call it good.

Building your AR-15 upper receiver

Now that we have our tools and our parts, let’s talk about assembly.

We’re going to assume you’re starting with an assembled receiver. Otherwise, you’ll need to install your dust cover and forward assist first (easy stuff, just follow the instructions that came with your dust cover).

Gas block preparation

To get started, I like to begin with getting my gas block assembled and attached to the gas tube, so that will all be ready to go later. You’ll notice on your tube you have a large hole on one side, and then a smaller hole that goes all the way through. 

That smaller hole is for the roll pin, and the large hole is what actually allows the gas to flow through and cycle the rifle. 

We want to insert the gas tube into the gas block so that hole faces downward towards the open center of the block, and the smaller hole lines up with the small hole on the side of the gas block, shown here.

assemble ar upper

Next, we simply use our punch to set the roll pin into that small hole, again making sure that the hole in the gas tube is facing downwards. 

ar upper build

Installing the barrel

Now we’re going to install the barrel. 

First up, we’re going to apply our grease or oil to the threads of the barrel nut and onto the barrel extension. This helps us get everything properly inserted and tightened down. Then we’re going to apply a little lube to the receiver threads and we’ll be ready to start assembling.

To install the barrel you start by sliding the index pin on top of the barrel into the slot on your receiver, which you can see below. This is how the AR-15 handles headspacing and proper chamber length.

Then you just slide the nut down onto the barrel with the threaded end facing the receiver. You may have a handguard nut that needs to go down first, depending on which kind you have. 

ar 15 upper build

Once you have the nut on, tighten it with your torque wrench and armorer’s tool (or the tool provided by your manufacturer). Usually this is between 40 and 80 ft lbs of torque. So if you’re not sure, aim for around 50 ft lbs.

building ar 15 upper

Installing the gas system

With the barrel nut on, it’s time to install the gas system. For this, we simply slide the gas block over the muzzle and line up the gas tube with the gas port on the receiver. Make sure the tube is parallel to the barrel and the block isn’t rotated to one side. 

building an ar 15 upper

Now, simply tighten the two set screws on the underside of the gas block, and your gas system is ready to go. 

how to assemble ar15 upper

Installing the handguard

Gather up your mounting hardware for your handguard. Your manufacturer should have instructions for what goes where. Some of these may also require a torque screwdriver.

Next, simply slide the guard over the barrel and attach it according to the manufacturer’s instructions. 

At this point, we’ve got the difficult bits behind us and it should be pretty smooth sailing going forward.

ar 15 upper receiver assembly

Installing the muzzle device

Your muzzle device will also have some installation instructions, but it is fairly simple. We’re going to put a little of our grease on the barrel threads and on the threads inside our device. Just a dab, no need to go overboard.

Then, slide the included crush washer onto your barrel threads with the cupped side facing the muzzle end. The washer looks a little like a bowl, and you want your device to basically sit in that bowl.

Now, simply follow the manufacturer’s instructions of how to properly time your device so that the holes that redirect the gas flow at the muzzle help to push the rifle in a direction that mitigates recoil.

ar 15 lower assembly instructions

With that, you’re basically done!

Final Steps: Installing the charging handle and bolt carrier group

ar15 lower assembly

If you’ve ever field-stripped an AR for cleaning, you know what comes next. Simply slide your charging handle into the groove at the top of your receiver, starting at an angle so that the tabs on the handle slide over the opening in the receiver. It works like a kitchen drawer slide, but upside down. 

Finally, we slide in our BCG, making sure the bolt is forward and the gas key (pictured below) is facing upwards and in the groove on the underside of the charging handle. That will slide forward and click into place, opening your dust cover in the process. 

ar 15 assembly

And congratulations! You did it. That’s all there is to it, besides adding optics, grips, and what have you. As far as function goes, your upper is now ready to rock and roll. 

assembling ar lower
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