The Ultimate Firearms Destination for the Gun Lifestyle

AR-15 Upper Receiver Assembly

Warning! When handling firearms, always observe safety rules and the precautions set forth in the firearm’s owner’s guide. Be certain that your firearm is unloaded and made safe before proceeding with this DIY.


Understanding how an AR-15’s upper receiver is assembled allows you to not only assemble a fully functional upper out of parts, but also gives you the knowledge to do preventive maintenance, to swap parts to keep your upper running right, or to customize your upper as you see fit.

With the current market being as hot as it is, many AR-15 enthusiasts are finding it increasingly difficult to find factory-assembled uppers available at the store. Although buying a complete upper from a reputable company comes with assurances that the quality and workmanship are all there — along with a great warranty in many cases — there is something to be said about building your own upper.

One of the advantages of rolling your own is selecting the exact specs and components you want. Another is gaining the experience and better understanding of how each part interacts with one another. Knowing how your gun operates can be as important as how to shoot it, as this insight can help you diagnose problems on your own, rather than relying on an armorer.

The AR-15’s upper receiver is where the bulk of the action occurs within the rifle. It is in the upper that accuracy and reliability are mostly determined. While there are many methods of assembling uppers that can make it quite an art form, we’ll leave all the tricks of the trade to the master gunsmiths. Within the scope of this article, our goal is to focus on the basics with this build.

AR-15 Upper Receiver Assembly
To keep things easier to follow, let’s go about this in sections. Parts for each of the sections will be called out at the beginning of each section.

I. Stripped Upper Receiver Assembly
II. Barrel, Gas System, and Handguard Assembly
III. Muzzle Device Timing
IV. Installing Bolt Carrier Group, Charging Handle, and Accessories
V. Function and Safety Inspection

Featured parts and specialized tools used in this article are sourced from Brownells. Part numbers and prices are highlighted in brown so you can easily find the items we used at Brownells’ website:

We want you to get as much out of this guide as you can, so we went ahead and disassembled the Daniel Defense A4 Upper Receiver Assembly (100-005-851 / $139.99) that the upper will be built off of. We’re doing this in case you have a stripped receiver and upper parts kit that you need to put together.

There are a few basic specialized tools that you will need to properly assemble an upper receiver.
1. DPMS AR-15 Upper Receiver Block
231-000-157 $45.99
2. Smith Enterprise .223 Armorer’s Wrench
851-000-068 $49.99
3. Loctite C5-A Antiseize Lubricant
532-000-012 $7.99
4. Brownells #3 Starter Roll Pin Punch
080-000-244 $6.99
5. Brownells Bench Block
080-000-493 $18.99
6. Break Free CLP
102-100-004 $6.99
7. ½-inch Torque Wrench
8. Vise
9. Hammer


I. Stripped Upper Receiver Assembly

AR-15 Stripped Upper Receiver
Forward Assist Assembly
Ejection Port Cover Assembly

1. We’ll begin with installing the Ejection Port Cover Assembly into the receiver. The hinge pin has a tiny snap ring installed on it. If your pin doesn’t have it installed, go ahead and do that now.

2. With the snap ring portion of the hinge pin to your right, feed the pin into the receiver’s right side ejection port cover hole going from right to left. While doing that, thread the pin through the port cover and spring.

3 & 4. Be especially careful with how the spring is aligned. The part with the longer leg should be on the right. The shorter leg needs to be twisted clockwise one full turn before the hinge pin is fully inserted into the receiver. Don’t let it unravel before the pin is fully inserted. It’ll take a bit of finesse to get this done, but it’s doable.

5 & 6. After the hinge pin is fully inserted, test that the port cover snaps closed and springs open. If it just flops open, the spring wasn’t installed correctly. If it looks good, let’s move on.

RECP-130500-DIY-07 RECP-130500-DIY-08
7. Now we’ll install the Forward Assist Assembly. Place the upper receiver block securely into a vise. Place the upper receiver onto the block. It’ll be easier to accomplish the installation by starting the roll pin into the receiver. Just get it started, don’t drive it too deep, otherwise it’ll interfere with the installation of the forward assist.

8. Now let’s take a look at how the forward assist needs to be oriented. The assist works by having its leg engage the teeth on the bolt carrier to help push it into battery. We need to make sure that the engagement surface is pointed toward the carrier when we install it.

9. With the spring installed on the forward assist, slide the assembly into the receiver’s forward assist opening, making sure the orientation of the assist is correct.

10. Now for some hand acrobatics. Depress the forward assist all the way in while driving in the roll pin with the roll pin punch and hammer. That spring will want to battle you, but you will win this battle.

11. Once you have the roll pin in a ways, the pin will hold the assist, making for easier hammering. Tap the pin until it is flush with the top of the receiver’s surface.

12. Check that the forward assist button springs back into place after pressing it. It should move freely without binding. Here is your completed upper receiver.


II. Barrel, Gas System, and Handguard Assembly

Barrel and Gas System Parts
Daniel Defense M4 Stripped Barrel
100-004-633 $176.95
High Standard AR-15 Barrel Nut
430-000-424 $10.23
Les Baer AR-15 Carbine Gas Tube
124-000-004 $14.99
Syrac Ordnance AR-15 Adjustable Gas Block (with Gas Tube Roll Pin)
100-008-858 $99.95


1. Take a look at the plugged end of the gas tube. You’ll notice that it has three holes. The larger one lines up with the gas port and the two small ones are for the gas tube roll pin.

2. Insert the gas tube into the gas block making sure that the roll pin holes are aligned and the gas hole is pointed downward, in line with the corresponding hole of the gas block. Make sure you see daylight out the other side of the roll pin holes.

3. You’ll also notice that there’s a bend in the gas tube; make sure that it goes upward toward the rear.

4. Using the roll pin punch and hammer, drive the roll pin into the gas block and tube while being mindful that the gas tube doesn’t shift position. The gas tube will want to shift around as you work, so be careful to adjust the holes back into alignment as you go.

5. After the pin is flush with the surface of the gas block, set the assembly aside for later use.

6. With the upper receiver assembly mated to the receiver block in the vise, take the barrel and insert it into the receiver. Be sure that the pin sticking out of the barrel extension is pointed up, at the 12 o’clock position.

7. The pin corresponds to a notch cut into the front of the threaded portion of the upper receiver.

8. Let’s move on to the barrel nut. The handguard we are installing uses a standard AR-15 barrel nut, so that’s what we’re using. Other handguards may come with proprietary barrel nuts. The basic principle of installation is the same, but follow the instructions that come with your choice of handguard. If you ever want to take the barrel off the receiver in the future, it’s a good idea to lube up the threads of the barrel nut so that it doesn’t seize. With a dab of antiseize on your finger, rub the compound so that it covers all the threads of the barrel nut. A light coat is fine, don’t gunk it on.

9. Hand-tighten the barrel nut onto the receiver.

10. Taking a look at it dead on from the front, you’ll see that the teeth of the barrel nut may not align properly with the receiver’s gas tube hole. You want a gap between the teeth to align with the hole, as this is where the gas tube will go.

11. Tightening the barrel nut onto the receiver is essential to seating the barrel correctly. Take the armorer’s wrench and attach the torque wrench to it.

12. Using between 30 to 80 lb-ft of torque, tighten the barrel nut so that one of the gaps between its teeth clears the gas tube hole on the receiver perfectly. You may find that it’s easier to do this in stages; torque the nut, then back it off a couple of times.

13. It is recommended to brace the wrench with your hand at the barrel nut so that the wrench doesn’t slip off. We also recommend using as little torque as you can because the more torque that is used, the harder it will be to remove later on and the more chance your receiver can get tweaked, but don’t go any lower than 30 lb-ft.

14. Don’t fudge this; the gas tube hole has to be perfectly clear of the teeth.

15. Now take the gas block assembly and slide it onto the barrel. The gas block will sit at the gas block shoulder that is cut into the barrel.

16. Some gas blocks may require a slight space between the gas block and the shoulder; others will not. You can verify this by measuring the gap between the shoulder and gas hole on the barrel and also the gas hole and end of the block on the gas block.

17. Depending on what type of gas block you get, some attach with set screws, some with pins, some require high-temperature thread treatment such as Loctite or Rocksett. Following the manufacturer’s directions, we tighten the set screws to their specs.

18. Make sure the gas tube does not come in contact with the barrel nut and that the gas block is tightened in place.


Handguard Parts
Troy Alpha Rail, 13-inch
100-009-696 $179.99


19. Following the manufacturer’s instructions, we install the Troy Alpha Rail.

20. Here is the upper with the handguard installed.


III. Muzzle Device Timing

Most muzzle devices need to be timed, or aligned correctly, to get the most out of the device. We’ll go through three common methods.
Shims and Peel Washers

Surefire MB556K Muzzle Brake Adapter
152-000-045 $134.10
1. Some muzzle devices such as this brake come with a shim kit that allow for it to be timed correctly on the barrel. The shims vary in thickness and are added onto one another to get the brake to the correct orientation. Peel washers work in very much the same way. Instead of adding layers of shims, you actually peel off layers of the washers until you get the desired result. A little heat added to a peel washer makes it easier to peel.

2. Following the instructions, we tighten the brake after installing the proper shims.

3. This brake is properly timed.


Crush Washer
DPMS A2 Flash Suppressor
231-000-198 $8.99
DPMS Crush Washer
231-000-175 $2.79
4. The most common way to attach a muzzle device is by using a crush washer. Here, we attach an A2 flash suppressor and crush washer to the barrel by hand.

5. The solid bottom of the A2 must face downward, but at hand tight, this one is over 180 degrees in the wrong orientation.

6. That’s OK, crush washers can be turned well over 360 degrees, so all we need to do is use the armorer’s wrench to turn it right side up.

7. This A2 flash suppressor is properly timed. Note that crush washers cannot be reused, so use a new one each time you reinstall a muzzle device.


No Washer
Smith Enterprise G6A3
Vortex Flash Hider
851-000-091 $59.99


8. The simplest of them all are devices such as the Smith Enterprise Vortex series that do not require timing at all. In fact, they discourage it.

9. The flash hider is hand torqued and will actually tighten itself with use.

10. This device is properly installed.


 IV. Installing Bolt Carrier Group, Charging Handle, and Accessories

Les Baer Custom AR-15 Bolt Carrier Group
124-000-001 $229.99
Phase 5 Tactical Ambi Charging Handle Assembly
100-009-955 $79.95
Magpul MBUS Gen 2 Sight Set
100-006-941 $89.99
1. Everyone has his or her preferred lube; we use Break Free CLP in this example. Whichever lube you go with, it’s important to know that AR-15s need to be lubricated, especially when new.


2. After lubing the charging handle, we continue lubing the bolt carrier group inside and out. A light coat will do for now.

3. Install the charging handle and bolt carrier group by inserting them into the upper receiver.

4. Following the instructions, we install the Magpul MBUS sights.

5. Our new upper is just about complete! All we need to do now is a final inspection.


V. Function and Safety Inspection

Now that the upper is assembled, we need to check that it is safe to fire and functions correctly.

Gas Tube and Carrier Key Clearance
Let’s first make sure that the gas tube and carrier key do not touch. If they make contact, it could cause the upper to shoot less accurately.

1. Remove the charging handle and bolt carrier group.
2. Replace just the bolt carrier group.

3. Gently push the bolt carrier group into battery. While doing so, see if you can feel or hear the carrier key make contact with the gas tube.

4. If it does, adjust the gas block until the gas tube does not make contact with the carrier key. All you should feel and hear is the bolt locking into the chamber.

Confirm Action
Now we want to check the action to make sure that the bolt cycles correctly.

5. With the charging handle and bolt carrier group installed in the upper, pull back on the charging handle to disengage the bolt from the chamber.
6. Cycle the bolt into the chamber by hand a couple of times, each time pulling the charging handle to disengage the bolt. Feel for any binding surfaces.
7. The action should be smooth and the bolt should lock up in the chamber securely. If you encounter binding or the action is not smooth, check your work or try adding lubrication and confirm the action again.

Checking Head Space

Forster Field Gauge
for .223 Remington
319-223-470WB $28.99

The beauty of the AR-15 is that it is a modular system and made according to a strict set of specifications. This means that most parts made by one manufacturer will fit parts from any number of other manufacturers. Headspace is important to check in all firearms, but because of the AR-15’s inherent design, most bolts and chambers will be within safety tolerances even when each are made by different makers, especially when the parts are new. However, headspace is a concern for AR-15s with heavily worn barrels and bolts.

The subject of headspace can be a lesson on its own, so we’ll just say that if your rifle’s headspace is not correct, it could lead to a catastrophic malfunction of your rifle leading to the destruction of your rifle or severe injury, even death. So spending the extra minute to check your upper’s headspace is well worth the effort.

8. There are several types of gauges made to check headspace; here we use a Field Gauge. Pull back on the charging handle to extract the bolt from the chamber.
9. Insert the Field Gauge into the chamber.

10. Push the bolt carrier group to close on the Field Gauge. Do NOT reassemble the gun and cycle the action on the gauge — just push gently on the back of the BCG by hand.

11. If the bolt does not lock up, then your upper is safe to fire. If the bolt does lock up, then your rifle is not safe to fire. The bolt, chamber, or both might be worn to the point that the rifle is no longer safe.

There you go; you now have an upper receiver assembly that you put together yourself. Attach it to a lower receiver and you’re ready for the range. Remember to keep the upper lubricated, especially during its first couple hundred rounds. Also don’t forget to double-check that any screws don’t come loose after your first outing with it.

With this newfound knowledge, you can now build, maintain, and modify an AR-15 upper receiver to your heart’s content. But please remember to follow all safety precautions and triple-check your work. Since we always love a dose of gun porn, share your creations with us and fellow readers on our Facebook page.



100-004-633    Daniel Defense M4 Stripped Barrel    $176.95
430-000-424    High Standard AR-15 Barrel Nut    $10.23
124-000-004    Les Baer AR-15 Carbine Gas Tube    $14.99
100-008-858    Syrac Ordnance AR-15 Adjustable Gas Block (with Gas Tube Roll Pin)    $99.95
100-009-696    Troy Alpha Rail, 13-inch    $179.99
152-000-045    Surefire MB556K Muzzle Brake Adapter    $134.10
231-000-198    DPMS A2 Flash Suppressor    $8.99
231-000-175    DPMS Crush Washer    $2.79
851-000-091    Smith Enterprise G6A3 Vortex Flash Hider    $59.99
124-000-001    Les Baer Custom AR-15 Bolt Carrier Group    $229.99
100-009-955    Phase 5 Tactical Ambi Charging Handle Assembly    $79.95
100-006-941    Magpul MBUS Gen 2 Sight Set    $89.99
100-005-851    Daniel Defense AR-15/M16 A4 Upper Receiver    $139.99


231-000-157    DPMS AR-15 Upper Receiver Block    $45.99
851-000-068    Smith Enterprise .223 Armorer’s Wrench    $49.99
532-000-012    Loctite C5-A Antiseize Lubricant    $7.99
080-000-244    Brownells #3 Starter Roll Pin Punch    $6.99
080-000-493    Brownells Bench Block    $18.99
102-100-004    Break Free CLP    $6.99
319-223-470WB    Forster Field Gauge for .223 Remington    $28.99

*Brownells’ pricing subject to change

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One response to “AR-15 Upper Receiver Assembly”

  1. Big Dave says:

    Not bad, but glaring barrel nut error. NEVER use an upper jig. Buy a Brownells or Geissele reaction rod and follow those instructions.
    Use the reaction rod for any and all muzzle devices too!!

    Barrel nut threads must be greased with AeroShell 33 MS grease. Barrel nut torque is 30 to 80 ft lbs. The lower the better. That is why it is paramount to pre stretch the threads AND bed the barrel extension. 35 ft lbs is ideal.

    MUST tighten and loosen the barrel nut about five times. This pre stretches the softer alloy threads on the upper receiver. Neglecting to do this can result in your whole upper and barrel integrity to work loose with vibration and time.

    Also, BED the barrel extension with Loctite 620.
    LOCTITE 620 is a high strength, high temperature resistant retaining compound. LOCTITE® 620 is a retaining compound designed for the bonding of cylindrical fitting parts. The product cures when confined in the absence of air between close-fitting metal surfaces and prevents loosening and leakage due to shock and vibration.

    Bed the gas block and your gas tube with 620 also. Use heat gun if you ever need to dissemble.

    Never put copper based never seize near really hot areas.
    Use a high temperature NICKEL based anti seize!
    Permatex 77134 Nickel Anti-Seize Lubricant, 0.5 oz Tube

    Building top tier AR15’s since 2008.

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