Orders ships next business day!

phone: 888-568-1771
How to Install an AR-15 Barrel & Gas System

How to Install an AR-15 Barrel & Gas System

Posted by 80-Lower.com on Mar 3rd 2020

Building an AR-15 upper piece by piece? Upgrading your barrel? Maybe you're swapping 5.56 or .223 for 300 Blackout like we are today. This guide illustrates how to install the AR-15 barrel and gas system. Total time required is approximately 30 minutes to one hour, and some basic tools are required or recommended. Let's begin!

Install Tools Required

  • Vise: Not pictured, you'll need a bench-top vise to secure the upper receiver while you tighten the barrel nut.
  • Upper Receiver Vise Clamp: Not pictured, you'll also need an upper receiver vise block clamp to prevent cracking or damaging the receiver in the vise.
  • Torque Wrench: You'll need a torque wrench with a 3/8" or 1/2" drive to properly tighten your barrel nut. The longer the wrench handle, the better. If your barrel doesn't provide torque specs, refer to U.S Mil-Spec. The M4 Armorer's Manual says to torque your barrel nut to 30 to 80 ft-lbs. (page 155). Realistically, 40 to 50 ft-lbs. is best. Most barrel manufacturers suggest this, too.
  • Barrel Nut Wrench: Required for installing and tightening the barrel nut with the torque wrench. Most barrels ship with one. If your barrel doesn't include one, be sure the one you buy has the right torque wrench drive attachment size (again, 3/8" or 1/2").
  • 3/32" and 5/32" Punches: Your gas block will likely include two to three roll pins: One or two for securing it to the barrel, and one for securing the tube inside. The roll pins typically measure 3/32" and 5/32". Get the right punches.
  • Gas Tube Alignment Tool: Optional. The alignment tool helps to ensure the gas tube interfaces with the bolt carrier group properly inside the upper receiver once installed. 

The Wheeler AR Armorer's Essentials Kit includes the tools listed above.

  • Breaker bar or Ratchet: You'll need to tighten and loosen the barrel nut (3) times to properly seat the threads. You should never break a nut loose with a torque wrench. Instead, grab a breaker bar or ratchet to do so.
  • Gunsmithing Hammer: Those punches are a PITA in the install, and if you've ever installed a lower parts kit, you already know. To make it easy, grab a brass/polymer-head gunsmithing hammer. This will prevent damage to your barrel while you tap the pins in.
  • Set of Allen Keys: Your gas block also probably includes (2) Allen-head set screws. These screws are meant to hold the gas block in place while you install the roll pins. Grab a set of Allen Keys to tighten the set screws.
  • Grease for Barrel Nut: Some say grease is optional. It is required. When you tighten the barrel nut three times, you'll wish you had grease. Not using grease is a bad idea, as you can gall or strip the threads.

New Install Video Available!

In addition to our detailed written guide here, we've now got an even more detailed instructional video that follows these instructions! Watch it below for further assistance and clarity on how to properly torque your barrel nut.

"Which barrel nut grease is safe/best?"

There seems to be a lot of inquiry about which grease one should use to install their AR-15 barrel. Keep it simple folks: U.S. Mil-Spec says to use Molybdenum Disulfide grease (page 143). The official name is Aeroshell 33MS. Any high-temperature, moly, or lithium grease that does not have metallic additives is safe to use. Do not use grease that includes graphite, as graphite can fatigue the aluminum threads on the upper receiver. We're using simple Tetra Gun Grease. 

AR-15 Parts Required

  • Barrel (w/ Extension): Virtually all AR-15 barrels ship with the extension already threaded and torqued. The headspace must be perfect, the index pin must align with the gas port perfectly, and torque requirements are very high, well over 100 ft-lbs. We don't recommend trying to install the extension onto the barrel yourself. We're putting together a 300 Blackout pistol upper for this install, but the installation steps and torque specs are the exact same if you're building a regular AR-15 rifle or pistol with a 5.56/.223 barrel.
  • Barrel Nut: The most important part in this install, the barrel nut secures the barrel to the upper receiver. Whether you're using a classic two-piece A2 or Picatinny rail, or a free-float unit, the barrel nut also acts as the mounting point for your handguard.
  • Gas Block: We're using the most popular gas block you'll find today, a low-profile block designed for a free-float rail. The "A2-style" FSB found on M4s or M16s is slightly different, and will not be covered in this install guide.
  • Block Roll Pins: The roll pins secure the gas block to the barrel, and secure the gas tube to the block.
  • Block Set Screws: Used for tightening the block to the barrel and holding it in place while you secure the block-to-barrel roll pin(s).
  • Gas Tube: Our gas tube is pistol-length to ensure our gun handles subsonic loads reliably, but your typical AR-15 build will likely use a carbine or mid-length tube. Install steps and parts required are still the same regardless of tube length or caliber.

The Install

Step 1: Slide Barrel Nut Onto Barrel

  • Parts: Upper Receiver, Barrel, Barrel Nut, *Handguard Attachment.
  • Tools: Allen Key or Allen Wrench (for Gas Block Set Screws).

Collect the upper receiver, the barrel and the barrel nut. If your barrel arrived with the gas block partially installed via the set screws, grab the appropriate Allen key (or wrench and socket) and remove the gas block by removing the set screw(s).

Next, slide the barrel nut onto the barrel, up against the receiver extension. Ensure the grooves of the barrel nut are facing the muzzle. The grooves are what allow the barrel nut wrench to capture and tighten the nut via your torque wrench.

*NOTE: You may need to first slide your handguard's attachment plate or handguard nut onto the barrel so it rests between the barrel nut and the upper receiver as shown above. Refer to your handguard's instruction manual. If applicable, ensure the gas tube hole in the handguard mount or plate aligns with the gas tube hole in the receiver.

Step 2: "Season", Tighten, and Torque Barrel Nut

  • Parts: Barrel Nut.
  • Tools: Vise, Receiver Vise Block, Barrel Nut Wrench, Torque Wrench, Grease.

First, apply grease to the receiver's barrel nut threads and the exterior of the barrel extension. The extension might be a tight fit. Seat the extension inside the receiver fully, ensuring the index pin on the extension is aligned and secured inside the cut-out notch in the barrel nut threads.

Next, hand-tighten the barrel nut to the receiver's threads. Check inside the receiver to confirm the barrel extensions's and receiver's feed ramps are properly aligned with each other. Ensure your receiver is secured in the vise and vise block clamp. It's imperative the receiver assembly does not flex, twist, or move while torquing the barrel nut. Our receiver is not clamped for the purposes of better illustration. 

Do not tighten the nut.


If you do not follow these instructions precisely, you will damage your torque wrench or nut wrench and fail to achieve a proper installation.

Best Practices for Tightening the Nut

  • Ensure the barrel nut wrench is parallel with the torque wrench's handle. If it is not, torque cannot be properly applied and the correct ft-lbs. will not be achieved when tightening the nut. The nut wrench could also warp.
  • Reference your barrel's manual for advertised ft-lbs. If a specification is not provided, default to 40 to 50 ft-lbs.
  • These steps apply to open-ended wrenches: When tightening the nut, with the wrench's teeth secured to the nut's top and side grooves (not underneath)always tighten from the left side of the receiver, in a downward motion. This pulls the wrench's teeth into the nut's grooves. Tightening from the right side of the receiver by pulling up on the wrench can bend the wrench outward, pulling its teeth out of the grooves, rendering the wrench useless. 

Your setup should look exactly like the wrenches, nut, and receiver pictured above.


Seasoning the Receiver's Threads

The barrel nut is made of steel. The AR-15 receiver's threads are made of aluminum, which is softer. Threading dissimilar metals together under high torque requires "seasoning" the threads of the softer metal by repeatedly loosening and re-tightening the nut. This ensures the barrel is properly seated and will not loosen under heat or pressure once torqued a final time. 

IMPORTANT: Do not loosen the barrel nut with the torque wrench. This can ruin the wrench's calibration. Instead, use a standard ratchet or breaker bar to loosen the nut. Ensure the breaker bar/ratchet handle and nut wrench are parallel when loosening.

IMPORTANT: When loosening the barrel nut, install the nut wrench underneath the barrel, in the bottom and side grooves of the nut. Pull up from the left side of the receiver. This again pulls the wrench into the nut's grooves rather than away, preventing the nut wrench from warping or bending outward.


Set your torque wrench to the the specified ft-lbs. 

  1. Torque the barrel nut with the nut wrench and torque wrench.
  2. Loosen the nut with either a drive ratchet/breaker bar.
  3. Back the nut off the threads, then re-tighten by hand. 
  4. Re-torque the barrel nut the specified ft-lbs.

Repeat these steps until the barrel nut has been torqued (3) times.

Step 3: Install Gas Block, Gas Tube, & Set Screws

  • Parts: Gas Block, Gas Tube, Set Screws.
  • Tools: Allen Key or Allen Wrench.

Collect the gas tube, gas block, and set screws. First, insert the end of the gas tube with the roll pin hole into the gas block. It is a tight fit; lubricant may assist. Press the tube into the block until the hole in the tube aligns with the roll pin hole in the block, as shown left. Slide the gas block onto the barrel, and allow the other end of the tube to feed through the gas tube hole (and hole in the handguard attachment, if applicable) into the upper receiver.

Grab the set screws from the gas block hardware and thread them hand-tight into the gas block with the Allen key or Allen wrench, securing the block to the barrel. While doing so, double-check the roll pin hole for the gas block-to-barrel pin is aligned with the cut-out for the pin in the barrel itself. The hole is circled in red above.

Step 4: Check Tube Alignment, Install Gas Block Roll Pin

  • Parts: Gas Block Roll Pin.
  • Tools: 5/32" Punch, Gunsmith Hammer, Gas Tube Alignment Tool or BCG.

Grab the gas block roll pin and 5/32" punch with gunsmith hammer. Once you seat that gas block roll pin, it ain't comin' out without a fight. Before you begin tapping it into the gas block, double-check alignment of the gas tube with the upper receiver. This can be accomplished with the Gas Tube Alignment Tool included in the Wheeler Armorer's Kit. Or, you can grab your BCG and charging handle, install the pair into the receiver, and check alignment via the handle and gas key on the carrier.

Once you've verified alignment of the gas tube and receiver, you may begin tapping the gas block roll pin into place. To prevent marring the finish on the barrel, seat the pin atop the hole and tap the pin directly with the polymer end of your gunsmith hammer. Once partially seated, continue tapping the pin into place with the 5/32" punch. The pin is longer than the gas block and barrel cut-out; ensure it protrudes at equal lengths on both sides. of the block.

Step 5: Insert the Gas Tube Roll Pin

  • Parts: Gas Tube Roll Pin.
  • Tools: 3/32" Punch, Gunsmith Hammer, Pliers (optional).

If you installed your own LPK in your lower receiver, then welcome back! The gas tube roll pin is about the same size and width as the bolt catch pin and yes, it can be a pain. This time, you're installing it on a rounded surface. It helps to re-seat your receiver horizontally in your vise for this one. Collect the 3/32" punch, gunsmith hammer, and roll pin and tap it into place. Take your time, it will fight you. Once installed, the pin should be flush on both sides of the gas block.

Your AR-15 Barrel and Gas System are Installed!

With the gas tube roll pin fully seated, your barrel and gas system install is complete! Perform a functions check of the BCG and charging a final time to ensure the gas tube is still aligned after seating the final roll pin. Once it checks out, you may proceed with completing the installation of your handguard, optics, and muzzle device. 

We'll see you on the range, deadeye.

DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At 80-lower.com, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.

We are a national retailer of individual components and not all products depicted on this website are legal in every state. Shipping of various products found on this website are prohibited to some states (such as California, Connecticut, District of Columbia, Hawaii, New Jersey, New York, Rhode Island, and Washington). The information, pictures, text or products presented on this website are not a representation by us, and should not be understood by you, that any product or completed firearm is legal to assemble or own in your state of residence. We encourage each and every builder to perform their own research about the state and federal laws that apply to them. It is your responsibility to understand the law and we encourage you to consult with an attorney or your local ATF representative.