If you’re building your own AR, you know that the bulk of the action takes place in the upper assembly. This includes the upper receiver, the barrel, and any attachments like a Picatinny rail handguard.
A lot of sites use the terms “upper receiver” and “upper assembly” interchangeably, so we’ll be doing the same, and looking at how to build a complete upper assembly.
To begin with, we’ll be assuming that you’re starting with a stripped upper receiver, as opposed to a “complete” upper receiver, which already includes the forward assist and ejection port door.
If you purchase a complete upper receiver, just skip ahead to the next step and you’ll be on track. Similarly, there are other steps you can skip if you purchased a pre-assembled barrel.
We’ve tried to cover all steps of the assembly as thoroughly as possible, so you can build yourself an upper assembly even if you’re starting with a just box of parts.
Before you build your AR upper, you’ll need to make sure you have all the parts you need. Keep in mind that the gas block, barrel, and gas tube must all be compatible, or the gun won’t be able to cycle properly. Other than that, most parts are more or less universal provided the chambering is a standard caliber.
Here’s everything you’re going to need:
- Upper receiver frame
- Forward assist and spring
- Ejection port door, roll pin, and spring
- Gas tube
- AR barrel and barrel nut
- Gas block
- Muzzle brake/flash suppressor
- Bolt Carrier Group (BCG)
- Charging handle
Strictly speaking, you don’t need a ton of tools to assemble an AR-15 upper, and most of them are pretty mundane. However, there are three tools that can make things much easier. A vice or upper receiver block – or both – can hold your receiver frame steady while you’re working.
A wheeler torque wrench makes it easier to install your gas block and can improve cycling efficiency. And a feeler gauge for the gas block ensures optimal spacing.
Here’s a complete list:
- Punch set
- Masking tape
- ½-inch torque wrench
- Anti-seize lubricant
- Needle nose pliers
- Wheeler torque wrench (optional)
- .025-inch feeler gauge (optional)
- Vice or upper receiver block (optional)
Step by Step Guide
Now that you’ve got all your parts and tools, it’s time to get started. Roll up your sleeves, put on some relaxing music – you’ll need it – and get ready to assemble your first AR upper.
Forward Assist and Ejection Port Door Installation
Assuming you’re starting with a stripped upper receiver, the first thing you’re going to need to do is to assemble the forward assist and the ejection port door. Thankfully, these parts are easy to install, so you’ll be finished with this step in no time.
To install the forward assist, first install the roll pin a fraction of the way, just enough so that it doesn’t fall out of the hole. Masking tape is a good idea here, to protect your receiver in case you miss with your hammer.
Insert the forward assist spring, along with the assist rod, and drive the roll pin the rest of the way into place. To test this, simply press the button and move the assist rod in and out to ensure that it moves smoothly.
The ejection port door is even more straightforward. Install the hinge pin through one of the holes on the receiver and one side of the door, then position the spring so the ends rest in the appropriate grooves on the door and the receiver. Hold the door and spring in place with one hand while you hammer the hinge pin into place with the other hand.
Gas Tube Installation
Many lower receivers come with the gas block and tube already attached. However, many don’t. If that’s the case, keep reading. If your lower already has a gas block attached, skip ahead to the next section.
To begin with, look at both ends of your gas tube. One end has a single large round opening, and the other has a horizontal slit with a small round hole in the tube wall.
Screw this end of the tube into your gas block, and use the locking pin to hold it in place. You want it to be positioned so that the hole in the tube wall will be facing the barrel side of the gas block.
Secure the other end of the gas tube to your receiver with the appropriate roll pin, and you’ll be ready to install the barrel. There’s no way to test the gas system without firing your gun, so be careful here that you’re aligning all parts properly. It’s easier to inspect everything now than it is to troubleshoot a jammed AR later.
Get your ½-inch torque wrench ready, because it’s time to do the most critical part of the assembly; attaching the barrel. Before you begin, apply anti-seize lubricant both to the inside of the barrel nut and to the threading on the barrel itself. If you accidentally screw it in too far, it’s critical to be able to back it out.
Now, torque the barrel down to at least 35 but not more than 85 foot-pounds. Basically, you want to get as close as you can to 85 foot-pounds without going over.
The trick here is that you need to end up with the gas block mounting holes in the general vicinity of the gas block, so you may need to do some tightening and loosening to get everything lined up and torqued down appropriately.
While you’re doing this, investigate the mounting hardware for your handguard. If it requires shims, read the instructions, and get them installed before you start messing around with the barrel. Once everything is all lined up, it’ll time to install the gas block.
Gas Block Installation
We know. Gas blocks are boring. Handguards are way sexier, and we promise we’ll get to them. But before you install your handguard, you’ll need to have your gas block installed.
First, let’s address adjustable gas blocks. There are many different versions, and each of them needs to be installed to its own specifications. If you have an adjustable gas block, go and read your manufacturer’s instructions for installation, and follow them. Then come back and finish this article, because we’re not finished yet!
If you own a standard gas block, you’ll need to ensure that it’s installed perfectly parallel to the barrel. Many barrels have pre-drilled indents to assist with alignment. If yours doesn’t, use a straightedge to mark the barrel and gas block with a pencil before you mount it.
No matter which method you use, we strongly recommend using Loctite on all the screws. This gas block is part of a pressurized system. The sudden stress of recoil will pull the screws loose over time if they’re not properly secured.
Bolt Carrier Group Assembly
Many receivers already come with an assembled bolt carrier group (BCG). However, even if your BCG came pre-assembled, you’ll eventually need to clean it, so it’s a good idea to understand how to assemble it for future reference.
To assemble your BCG, slide the bolt into the carrier, making sure to keep the fin on the top aligned with the slot in the carrier, so it can be seated all the way. Next, insert the cam pin through the bolt, being careful to bypass the carrier gas key. Pull the bolt out as far as you can, then rotate the cam pin by 90 degrees.
From there, you need to insert the firing pin, skinny side down, from the front of the carrier group. Hold the firing pin in place and insert the retaining pin. To ensure that the firing pin is locked in place, tug on it, and rattle on the bolt. If everything remains in place, your good to install your BCG.
BCG and Charging Handle Installation
Your BCG and charging handle get installed together, so we’ll address them as a single unit.
First, you’ll need to partially install your charging handle. To do this align the bumps on the handle with the cutouts in the receiver, and insert the charging handle partway, about an inch deep.
Next, ensure that the bolt-on your BCG in pulled out all the way, and push the BCG into your receiver as far as it will go. Press the bolt forward, and the entire BCG should lock into place. Push the charging handle in the rest of the way, insert the locking pins, and you now have a functioning rifle.
If you want to test fire your gun, you can do it at this point, although we wouldn’t recommend shooting for an extended period of time without a handguard installed. Still, a test shot or two can help you make sure your gas system is properly installed, and your action is cycling the way it should.
Once you’ve tested everything, it’s time to complete your build.
You’ve stuck with us so far, and now it’s time for the most fun part of any AR: the handguard. To begin with, we hope you read our barrel instructions thoroughly and installed any shims required by your handguard’s manufacturer. This is essential for most free-floating handguards.
Provided your barrel nut shims are properly installed, mounting your handguard is a cinch. Line up the holes, tighten the screws, and you’re good to go. That said, you want your handguard to be as straight as possible.
This is a precision machine, and even a fraction of a degree of error can make it harder to align sights and other accessories. Tighten your screws in an alternating pattern, like lug nuts, and your handguard should be perfectly aligned.
One more important thing is to use Loctite on all of these screws, just as you did with your gas block. This will ensure that your handguard stays where you put it, and doesn’t start rotating around during a day at the range.
Muzzle Brake/Flash Suppressor Installation
Finally, you’ll want to install any barrel-mounted accessories you may be using. These are strictly optional features for many people, but most ARs utilize one or the other. In some states, on some barrel lengths, they’re not only mandatory but must be pinned and welded to prevent removal. Read up on your local laws to make sure your gun is legal.
Your muzzle brake or suppressor will screw on to the end of your barrel and may require a crush washer or shims depending on how far the barrel screws into the device. As a general rule, muzzle brakes will be oriented with their slots parallel to the ground, to reduce recoil and stabilize the barrel.
Flash suppressors will work differently depending on their design. For all devices, always make sure to read your manufacturer’s installation instructions to make sure you’re attaching them properly.
Another potential barrel-mounted accessory is a sound suppressor or silencer. These are a great way to reduce the noise from your gun and protect your hearing. If you want one, be prepared to wait; you’ll need to apply for an ATF tax stamp, and those take over nine months to get approved.
As we mentioned at the beginning, the upper receiver and the rest of the upper assembly are where most of the action happens on an AR-15. This is where you want to be looking for quality parts, like match-grade barrels, low profile gas blocks, and powerful, recoil-reducing muzzle adapters.
But why stop there?
Even once your AR is built, it’s never really finished. You can add a reflex or red dot sight for 3 Gun competitions or home defense. You can attach a traditional scope, and use your AR for hunting. There are also plenty of flashlights, lasers, and enhanced iron sights you can use to make your AR truly your own.
And that’s the beauty of the AR-15 as a platform. You don’t just own an AR. You own your AR. So build yours as best you possibly can. We hope our guide has helped you along the way.
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