Assembling a functioning AR-15 upper receiver from nothing more than a box of parts is one of the most rewarding experiences of gun ownership. There’s something deeply satisfying about building an upper yourself, not to mention educational. But, it can also be exceptionally frustrating when things go wrong.
To help make sure your first upper goes “bang” at the end of the day instead of “click,” we’ve put together a handy guide that will walk you through the entire process, step by step.
Before You Build: Parts & Tools
Before we get into the process of actually assembling an AR upper, it’s important to make sure that you have everything you need to get the job done.
- Upper Receiver—You can buy a stripped one and install the forward assist and ejection port cover yourself, but it’s often cheaper and easier to buy one complete. For this guide, though, we’ll assume you’re using a stripped receiver.
- Forward Assist—This should come with all the small parts necessary for installation, like the spring and roll pin.
- Ejection Port Cover—As above, this should come with its related spring and pin.
- Gas Block—If you think you may want to tune your AR-15 at any point or use it with a suppressor, we recommend installing an adjustable gas block from the start
- Gas Tube—Make sure this is matched to the barrel. If your barrel is designed for a midlength gas system, you’ll need a midlength tube.
- Muzzle Device—This can also double as a suppressor mount, in some cases.
- Handguard—In most cases, these will come with a proprietary barrel nut. If you’re using an old-school clamshell handguard, you’ll need a few additional parts, like a barrel nut and delta ring assembly.
- Bolt Carrier GroupCharging Handle
Once you’ve got your components together, it’s time to go through the checklist of tools. It’s worth noting that some of these are optional; they make the process easier or faster but aren’t strictly necessary.
- AR-15 Upper Block—There are a lot of good models out there. Some are simple Delrin blocks that hold your upper receiver. Others are more involved and may also support the chamber. Depending on which model you purchase, you may also need an upper receiver rod or similar tool to safely install your muzzle device.
- Vice—It may be tempting to try to do this without a vice. Don’t. A vice is absolutely necessary to hold the upper securely in place while you torque down the barrel nut and other components.
- Torque Wrench—This should have a 1/2″ drive and be adjustable from 30-80 ft-lbs.
- Torque Driver—We like the Wheeler FAT Wrench, but any torque driver with interchangeable bits will do.
- Punches—At a minimum, you should have a decent standard punch set. Ideally, you’ll also want roll pin punches to drive the roll pins without mangling them. Having punches in materials other than steel can also be useful if you’re sensitive about marring your finish.
- Hammer—Obviously, if you need punches, you’ll need a hammer to drive them. A hammer with variable heads allowing you to select a steel, brass, or polymer striking surface will similarly help you avoid marring anything.
- Needlenose Pliers—These are handy for many tasks, most notably holding roll pins and installing the snap ring on the dust cover.
- Barrel Nut Tool—Usually, this will come with your handguard and barrel nut. If you have a combination AR-15 Tool, that will usually also have a standard barrel nut tool built in.
- Feeler Gauges—You’ll want to have a set of feeler gauges in order to properly space the gas block.
- Headspace Go/No Go Gauges—you may have heard that AR-15s are self-headspacing and don’t require these gauges. While headspace is rarely an issue in AR-15s, the results of improper headspace are dangerous enough that it would be irresponsible for a builder to forgo checking.
- Grease— Required for the installation of the barrel and gas block. There’s some debate as to what grease is best for this application, but we’re partial to lithium-moly.
- Bench Block—This one is entirely optional but extremely convenient. A bench block essentially works as a jig to hold components in place while you work on them and makes tasks like driving in a gas block pin frustration-free.
Assembling the Upper
The first step in AR-15 upper assembly is to install the forward assist. Start by mounting your stripped upper in your vice using the upper block. Then, place the forward assist assembly (return spring in place) in the appropriate opening on the receiver. The part of the forward assist that contacts the bolt will have a slight curve to it—make sure that curve faces in toward the center of the receiver.
With the forward assist in place, press it into the receiver slightly and align it so that you can see through the roll pin hole. You can then use a punch (not the one you’ll use to drive the roll pin) as a slave pin to keep the forward assist in place. Drive the roll pin in to retain the forward assist.
Next, we’ll move on to the dust cover. The dust cover assembly is comprised of three or four parts, depending on whether or not your snap clip comes preinstalled on the hinge pin. If it’s not preinstalled, then you’ll need to press it into the indented ring in the hinge pin. This can be done by holding it in place with pliers and tapping it in with a hammer or just squeezing it into place using a set of standard pliers, although the latter method brings with it a greater risk of shooting the clip across the room.
With the snap ring in place, you can slide the hinge pin through the receiver and dust cover halfway, stopping just before the pin protrudes into the open section in the middle of the dust cover. This open section is where the dust cover spring goes. Place it there now, with both legs of the spring pointing up. The longer leg should be toward the “muzzle,” even though there isn’t a muzzle there yet.
Hold the spring in place so that the short leg is tensioned against the receiver and twist the long leg down 180 degrees, turning it towards the receiver, until it lays flat against the dust cover and applies pressure against it. Slide the hinge pin the rest of the way through the dust cover.
Now is a good time to test the dust cover and forward assist to make sure they work properly before moving on to the next steps. The forward assist should spring back after being pushed in. The dust cover should do the same unless closed fully, in which case it should lock into place.
Installing The Barrel
The next phase of building an AR-15 upper is to install the barrel. Before you go and press it into the receiver, though, it’s a good idea to check the headspace.
To check the headspace on your bolt and barrel, remove the bolt from your BCG, then strip it, removing both the ejector and extractor. Start with the Go gauge by placing it into the barrel and then inserting the stripped bolt into the chamber. You should be able to fully insert the bolt and twist it to lock the lugs. If you cannot, your barrel has insufficient headspace.
Next, repeat the process with the No Go gauge. This time, your bolt should not be able to fully insert and lock. If it can, your headspace is excessive. Should your bolt and barrel combination fail either test, you’ll need to replace one or the other before proceeding with your build.
For these next instructions, we’ll assume you’re using some variety of free-floating rail. If your build involves a clamshell handguard and a delta ring, there are some additional steps you’ll need to take that won’t be covered here.
Insert the barrel into the upper receiver so that the indexing pin on the barrel fits into the slot in the receiver. Next, apply your lithium-moly grease or anti-seizing compound to the threads of the receiver. This step is important; don’t skip it, or you could risk damaging your upper. Make sure the threads are fully coated with grease or compound, but you don’t need to go overboard. A thin layer will do.
Now it’s time to install the barrel nut. Thread the nut on hand-tight and attach your barrel nut tool to your torque wrench. Torque the nut down to 30 ft-lbs.
This next part may seem tedious, but it is important. Break the nut free and back it off. It’s not technically necessary, but you should use a breaker bar for this instead of your torque wrench. Using a torque wrench to break the nut free can reduce its accuracy over time if done repeatedly. Once the nut is backed off, torque it down again. Then repeat the process one more time.
The objective here is to mate the threads on the receiver and barrel nut, allowing you to torque the nut fully without damaging either. In total, you should have installed the nut and torqued it to 30 ft-lbs 3 times. After the third installation, continue torqueing the barrel nut until is aligned with the gas tube opening in the receiver. It’s not often an issue, but don’t exceed 80 ft-lbs of torque on the barrel nut.
Double-check the alignment using a gas tube gauge, or just using the gas tube itself.
Next, we’ll move on to the gas block. At this point, most gas blocks are clamp-on or screw-retained designs, so that’s what these instructions will cover; if you’re using a pinned gas block, defer to the instructions that came with it for installation.
Before installing the gas block, pin the gas tube into place. This is as simple as inserting the tube into the block, aligning the holes, and driving in a roll pin. If you have a bench block, this process is exceptionally simple; if not, the trickiest bit is usually getting the gas block to sit straight while you drive in the pin.
Now, we’ll install the gas block onto the barrel. The gas block must be properly aligned with the gas port in the barrel for your rifle to function.
There are a lot of different tricks armorers use to accomplish this, but our favorite is to make a mark down the center of the gas block with a pencil. You’ll want to use a ruler or carpenter’s square to make it straight. Then, make another line along the barrel directly through the gas port and about 2-3 inches past it, again using a ruler.
Slide the gas block on and slot the gas tube into the opening in the receiver, using the lines to keep the gas block straight. You’ll want the gas block to sit .025 inches away from the face of the shoulder on the barrel for perfect alignment, so break out the feeler gauges before tightening down the block. If you’re using a clamp-on block, take care that the block doesn’t shift during tightening. For a screw-retained block that matches up with dimples in the barrel, this isn’t usually as much of an issue.
The exact torque rating on your gas block can vary between manufacturers, but it’s usually somewhere around 30 in-lbs.
Handguards and Muzzle Devices
Next comes the handguard, but unfortunately, we can’t offer a great deal of guidance here. Free-floating handguards usually use proprietary mounting techniques, ranging from clamping, to cross bolts, to direct bolts right into the handguard. For this, you’ll want to defer to the instructions that came with your handguard.
After your handguard is in place, it’s time to tackle the muzzle device. Again, instructions will vary depending on which device you are installing, so always consult the manufacturer’s directions before proceeding. With that said, most muzzle devices currently on the market use one of two attachment systems.
The first and simplest method is a crush washer—this is how a standard A2 compensator is installed. For this method, simply place a crush washer on the barrel, thread on the muzzle device, and torque the device into place using the appropriate wrench for that device.
Like handguards, many muzzle devices come with a proprietary tool for this procedure. Others can be installed using an armorer’s wrench, or in the case of certain muzzle brakes, a screwdriver placed through the ports. No particular torque rating is usually required with a crush washer.
Another common method utilizes shims (and sometimes a spacer to achieve a perfectly square shoulder) to retain the muzzle device. For this method, install the spacer if one is provided, then try threading on the muzzle device. Ideally, it will stop just a little short of proper alignment relative to the rifle, about an eighth of a full turn.
If it does not, back it off the threads and add a few shims, then reinstall the device and check alignment again. Repeat this process, adding and removing shims as necessary until the muzzle device stops as described in the paragraph above. Then, using your torque wrench, torque the muzzle device down to the specification prescribed by the manufacturer.
With both of these methods, threadlocker like Rocksett or Vibra-Tite can be used for additional security, although it’s not usually necessary unless you plan to use your muzzle device as a suppressor mount.
With the muzzle device in place, all that’s left to do is add a bit of lubricant to your charging handle and bolt carrier group and slot them into place.
If you used any threadlocker, whether in the installation of your muzzle device or handguard, you’ll want to give it 24 hours to cure before test-firing your new creation. Otherwise, your upper is now complete and range ready.