How To Build An AR-15 Upper [Hands-On Guide]

CAN WE BUILD IT? YES, WE CAN!

Building an AR-15 upper is not too hard, but a good guide will go a long way in making sure it is done right. That’s what we are here for!

From the tools to the build, we got you covered.

IS BUILDING RIGHT FOR YOU?

The AR-15 is somewhat rare in that it’s pretty easy to assemble at home. This gives you a lot of flexibility and options into what goes into your upper. But it also means you’re responsible for your own quality control, and you might need some tools you don’t currently own.

While an AR-15 lower can be assembled with no more than a table, a punch set, and a hammer — the upper isn’t as easy. It’s fewer parts and quicker to throw together when you know what you’re doing, but it does require more tools and less common tools.

However, as long as you’re willing to spend the money on a few extra bits and pieces, building an upper is pretty easy. 

TOOLS

While it is technically possible to torque a barrel without a vise, I really recommend getting a vise. Any of the super cheap 4-inch versions on Amazon will be fine, I’ve built probably 20 plus uppers using a $35 vise. The downside is that any vise able to hold your rifle enough to torque a barrel on will also need to be mounted to the table. This generally means drilling the table and using some bolts.

You can’t just throw your upper in the vise without damaging it. There are a few ways to skin this cat, but I recommend a reaction rod. Geissele makes a good one, as does Wheeler. I use the Real Avid one and have done so for years. These are nice because you have options for mounting and turning the action as you work. If you want something cheaper, a vise block that clamps around the action is only $20 normally and works well. It does limit you and makes assembly a little harder, but it’s cheap.

Technically speaking, the only correct lube is AeroShell 33MS. That said, I use White Lithium grease. You NEED to use grease on the threads for the barrel nut and muzzle device, or you risk ruining them. When selecting a grease, the major thing you need to avoid is a grease with graphite or copper in it. AeroShell 33MS is the mil-spec grease, and I would assume it’s the mil-spec grease for a reason, but I honestly don’t know what that reason is.

I’ve looked, I’ve asked, and no one has been able to tell me why it is the mil-spec other than that it doesn’t have graphite or copper in it. Personally, white lithium is a lot more universal, and buying a tube of that makes more sense to me. I started using white lithium based on the advice of a major brand, and I’ve never had an issue, even if it isn’t “mil-spec.”

Any foot-pound torque wrench that fits your barrel nut’s wrench size will be fine. Harbor Freight is great for this if you have one near you.

Tools You (Maybe) Need

One problem with writing a guide to building an upper is that I don’t actually know what tools and processes you’ll be doing. There are almost as many ways of attaching a handguard as there are handguards. A lot of brands use some common methods, but almost all of them are slightly different. Many are radically different, even within the same brand but across product lines.

Depending on the handguard you choose, you might need hex keys or Allen keys, or no keys. Maybe you’ll have the tool supplied with the handguard, maybe you won’t. Maybe you’ll need a crow's foot for the barrel nut, maybe they’ll sell you a wrench head separately, maybe they will include it with the handguard — I really don’t know ahead of time.

These are all good tools to have on hand no matter what, but if you want to go a minimalist route, you’ll have to research your handguard ahead of time and plan accordingly or just wait until you open the box and then hit the store after.

Something a little more specialized you might need are crush washers for the muzzle device. Not all muzzle devices use a crush washer, but most of them do. Some include one in the box, some don’t. Thankfully, if you need one, they are cheap and easy to get.

If you plan on pairing this build with a suppressor, you'll want shims for your muzzle device instead. More on this later, but check with your muzzle device manufacturer for what they recommend.

HOW TO BUILD AN AR-15 COMPLETE UPPER

Building an upper will be more or less the same, with some details changed, for almost any normal-ish AR-15 upper on the market. If, while following this guide, you realize that your handguard or barrel nut works differently, stop. Google it, and look for specific instructions for your part. Don’t force things. Don’t crank on something and hope it fits. Take your time.

I’ll be putting together a Wilson Combat complete upper. Every part on this build was made by WC except the muzzle device because I forgot to ask for one. My muzzle is a generic A2 flash hider of some mystery brand that I found in my parts box.

Wilson Combat was nice enough to supply the parts for this build, and they even painted it with their Armor-Tuff OD Green coating. I have it say, it came out looking beautiful. Big thanks to them for helping out with this guide!

First Steps

Step one is to inspect your parts and make sure they are clean. Barrels and barrel nuts, especially, tend to be shipped with preserving oil or grease on them that is often very sticky and icky. For this build, only the barrel nut really needed to be cleaned, but it really needed to be cleaned. A quick spray of Simple Green and a rinse-off in the sink, and it was good to go. 

You want to remove these packing greases because they really aren’t built to help the gun function. They do great for keeping the parts from rusting during storage and shipment, but they can cause problems during assembly and especially during test firing. 

I also cleaned out the bore of the barrel with some Hoppes and a BoreSnake just in case there was packing grease in there.

Be sure to let these parts dry completely before assembly. 

The Barrel

Barrels for the AR-15 are held to the upper receiver via a barrel nut. You might think the barrel nut normally comes with the barrel, but you’d be wrong. Instead, it is normally a part of the handguard. 

Most barrel nuts just slide over the barrel and thread onto the receiver. Some, annoyingly, use a jam nut or something else to make life harder. But 9 out of 10 or more just thread on.

Wilson Combat’s system is fairly typical. Place the barrel in the receiver so that the small pin on the top of the barrel extension fits into the cut-out in the receive extension. 

Slide the barrel nut over the barrel and tighten it down just hand tight. As long as it fits well and there are no weird burrs or anything else, unscrew the nut all the way.

Apply a small amount of grease to the threads of the receiver, I use about two grains of rice amount of White Lithium. 

Screw the nut back on all the way, but again, just barely finger-tight. Unscrew it and re-screw it at least three times. You’re just trying to make sure the grease is even throughout the threads.

Most barrel nuts will need to be timed in some way. Sometimes that means making a hole in the barrel nut overlay the gas tube hole in the receiver. Sometimes, that means getting the right screw holes in the right place so the handguard can fit. For Wilson Combat, it’s the latter. 

Most barrel nuts need between 35 and 100 ft.lbs of torque, but the exact amount varies by manufacturer. Wilson Combat quotes 35 to 50. Using between that amount of torque, I need to time the nut so that the gas tube hole in the receive is between the handguard attachment points in the barrel nut. WC also gives you a shim in case you need it.

For me, I did need it, and it took about 40 ft.lbs to time correctly.

Read over your handguard’s owner’s manual for the torque values and any special instructions for the barrel nut system.

Gas System

I’m going to be talking about low-profile gas blocks because they are by far the most common people to use. If you’re using a Front Sight Block gas block, you’ll need a different guide because I’ve never installed one.

Most gas blocks work basically the same way. They slide down to the gas block shoulder on the barrel and then are held in place by set screws. Some barrels have dimples for the set screws, many do not. Some gas blocks clamp on from the sides instead of screwing up to the barrel. 

It’s not a big deal either way, just look at your gas block and figure out how the screws work.

For Wilson Combat, and really most gas blocks in general, they screw up to the barrel.

Take a close look at your gas block and figure out the front and backside. The front will have a small hole going across from side to side of the gas block, see the image below.

That is for the roll pin that will connect the gas block to the gas tube.

If your barrel has dimples for the set screws, you need only slide the gas block onto the barrel with the front of the gas block facing toward the muzzle of the barrel, overlay the screw hole and the dimple, and set the set screws.

Assuming your barrel was made correctly, the dimple will align the gas block with the gas port, and all will be well.

If you don’t have a dimple, slide the gas block back until it touches the shoulder of the barrel. Slide it forward of the shoulder by about the width of a credit card, using an actual credit card helps. This should line things up. One small step of QC that you can take is to shine a flashlight down the muzzle of the barrel. If you can see the light reflecting through into the gas block, the hole is lined up. Some gas blocks have a hole drilled down the top of the block so you can see the gas port during installation, checking that helps a lot if you have one.

For now, tighten the set screws until they are firmly tight but don’t go nuts on them. Eventually, you will want to add Loctite, but I normally do that after test-firing the rifle and making sure the gas system works.

Depending on your gas length and the design of the barrel nut, you might need to place your gas tube in the receiver before attaching the gas block. For my build, I was able to do the gas block first and then add the gas tube.

At one end of the gas tube, it will be just a tube. At the other end, you will see three holes. One will be fairly large, and two will be on either side of one another and fairly small, see the image.

The large hole goes over the gas port, and the two small ones are where the roll pin will go.

Make sure the gas tube is facing the right way, and then insert it into the receive before inserting it into the gas block.

Line up the roll pin holes and drive in the roll pin.

This part can be a huge pain in the ass. I’ve found that if I use the reaction rod vise, I can normally do this using a pair of Real Avid tweezers to hold the roll pin, my hand to support the barrel, and my other hand to hold the tap hammer to get the roll pin started.

Once the pin is started, a small wide pinch is handy to get it flush with the gas block.

Take your time, and try not to curse too much. 

Handguard

Like barrel nuts, handguards come in a lot of flavors. Some use a few screws to clamp onto the barrel nut, some use screws directly into the nut, and some use weird and complex systems.

Wilson Combat is pretty easy and just screws into the barrel nut itself with four screws. All you have to do is slide the handguard on, line it up, and screw the screws.

This is likely a step you will need to look at your owner’s manual if the process is proprietary. 

Muzzle Devise

Most common AR-15 muzzle devices just screw on with direct threads and use a crush washer for timing and torque. If you have something self-timing or more complex, read the owner’s manual. 

If you're going to mount a suppressor, you'll need precision shims. Crush washers are fine for normal muzzle devices but can induce concentricity issues when you add something like a suppressor to the mix.

For me, I used a simple A2 flash hider. Throw the crush washer on the barrel, apply anti-seize grease to the threads, hand tighten the flash hider on and off the threads three or four times to spread the grease. Then hand tighten the flash hider, and look at the timing.

Mine ended up almost timed with just hand-tight fitting. Had I turned the flash hider a little more with a wrench, it would have technically been timed. But this would have had very little pressure because it took so little timing.

Instead of turning it about 20 degrees, I turned it 380ish degrees so that it made one full turn past the correct timing.

Crush washers are normally designed to give you two or more complete rotations of timing, so you shouldn’t worry about just one turn. Personally, anything less than a half turn, and I’ll do it one full turn plus the amount of timing required.

You might be wondering, “Does the crush washer face forward or backward?” It won't matter. Some manufacturers tell you taper facing forward, some say backward, and some say it doesn't matter. I don't think it matters, and I've never had one come off or come out of time.

Charging Handle & BCG

These require no real assembly. Just clean them off, lube them, and install them like normal. The charging handle goes in first, followed by the BCG. Make sure the bolt is forward and unlocked.

LOOSE ROUNDS

I hope this guide is helpful, but I understand that due to the limitations of there being so many options on the market, there are some questions I can’t answer for you.

Building an upper isn’t too hard if you have the tools and at least a general sense of what to do. And that, at least, I hope this article will provide you with.