Upper Receiver Assembly (AR-9, AR-15, and AR-10/LR108)

Step U5: Gas System Installation

Objective: In Step U5 of upper receiver assembly, we will install the gas system (gas tube and gas block).

NOTE: This step does not apply if you are building an AR-9 on the Aero Precision EPC platform, which operates on direct blowback principles, rather than direct impingement.

Tools Required

1: We have tried a few gas block jigs and have yet to find any that fit a wide range of gas blocks without modification. We have exhausted our retail options, so we are getting ready to try out some options available on Etsy and will report back soon.

2: If not using a borescope and gas system illuminator to align the gas port and gas block.

Components Required

1: Gas tube roll pin is usually included with the gas block and gas tube (you will probably get one with each). All of the products that we recommend in our kits include the required gas tube roll pin. However, if you select another gas tube and gas block which both do not include the roll pin, you will need to purchase a gas tube roll pin^$ separately.

Instructions:

Step U5.0: Remove Existing Gas System (replacement/upgrade only)

If you are replacing your gas block or gas tube (or need to remove the gas system for any other reason), first ensure the firearm is unloaded.

If you need to remove the gas block from the barrel, you will need to remove the handguard first and you will most likely need to remove the muzzle device (unless you use a low profile muzzle device like the VG6 Epsilon SL muzzle brake).

Back out the gas block set screws (or loosen the clamp screws, if a clamp on gas block) and carefully slide the gas block and gas tube off of the barrel.

If you are reusing either the gas tube or the gas block (but not both together), you will need to punch the roll pin out of the gas block using a 5/64″ roll pin finish punch.

NOTE: Do not reuse roll pins.

Step U5.1: Dimple the Barrel

If you are installing the gas system on a new barrel (or one that has not been dimpled), we strongly recommend that you dimple the barrel to prevent shifting of the gas block over time (misalignment with the gas port on the barrel will cause cycling issues).

Identify the set screw spacing for your gas block. Below are the measurements for the products that we regularly recommend and use.

*** Note that there is no dimpling jig that supports this span. We recommend at least dimpling the barrel for the screw that is closest to the shoulder (closest to the chamber; closest to the gas port).

Secure the barrel into a vice with the gas port facing up. Use a barrel vice block, padded/non-marring vice jaws, or wrap your barrel to avoiding marring the barrel finish.

Install the dimpling jig onto the barrel according to the manufacturer’s instructions. The instructions below are for use of the recommended SLR Rifleworks Barrel Dimpling Jig^$ or the KAK dimple jig^$.

Locate the set screw hole marked with the applicable spacing for your gas block. Slide the jig onto the barrel with the set screw hole toward the rear of the barrel (away from the muzzle). Roughly align the screw hole on the jig with the gas port on the barrel. Thread the alignment set screw into the jig and drive until it contacts the barrel. Wiggle the jig as you gently tighten the set screw. You should feel the point of the set screw engage the gas port. DO NOT OVER-TIGHTEN. You only need to tighten to the point that the jig is securely indexed to the gas port. If you over-tighten, you will damage the gas port.

Once the jig is installed, loosen the vice and rotate the barrel 180 degrees so that the gas port is facing down. Install the appropriate drill bit into a hand drill or drill press. Insert the tip of the bit into the first hole in the jig and drill a slight depression into the barrel. DO NOT DRILL TOO DEEP (this is barrel dimpling; this is not barrel perforation). Repeat for the other hole.

Remove the jig by loosening the set screw and sliding off the barrel. Your barrel should now have 2 shiny silver depressions on the side opposite the gas port.

Step U5.2: Assemble the Gas Tube into the Gas Block

If you are using an adjustable gas block, adjust the orifice to the full open position, per the gas block manufacturer’s instructions.

Remove the set screws, if applicable.

Identify the front end of the gas tube (plugged end). The gas tube must be oriented such that the larger hole faces the gas port on the barrel.

Identify the rear of the gas block. The gas port hole will be drilled closer to the rear of the gas block.

Insert the gas tube into the gas gas block (from the rear) with the larger hole facing down (toward the gas port).

Align the gas tube roll pin hole in the gas tube with the corresponding hole in the gas block. It may help to use your roll pin punch to verify alignment. Once aligned, verify that you can see the gas port opening on the bottom of the gas tube through the hole in the gas block, as viewed through the rear set screw hole (if possible).

Install the gas block into a gas block jig.

Insert a new gas tube roll pin into the starter punch and begin to drive the roll pin into the gas block. As the roll pin is driven into the gas block, you may need to adjust the gas tube slightly to ensure alignment.

Once the starter punch bottoms out, switch to a 1/4″ steel punch. Drive the roll pin until almost flush with the gas block.

Switch to the finish punch and drive the gas tube roll pin until it is flush with the gas block.

Step U5.3: Mark the Barrel and Gas Block (if not aligning with borescope)

Insert the gas block alignment tool into the gas block according to the manufacturer’s instructions, ensuring that the gas port on the gas block aligns with the applicable feature of the tool. Using a pencil, mark the front edge (gas port is closer to the rear) of the gas block on the tool on either side of the center slot. Mark the front face of the gas block at the center slot of the tool. Remove the tool from the gas block.

Snap the gas block alignment tool onto the barrel with the hole toward the rear of the barrel. Align the feature on the tool with the gas port on the barrel. Using the mark applied to the tool, transfer the mark to the barrel to mark the appropriate depth and center line.

Step U5.4: Install the Gas System onto the Barrel

Slide the gas system onto the barrel, with the gas tube pointing to the rear.

Feed the gas tube through the applicable hole in the upper receiver (Aero Precision Enhanced upper) or barrel nut (threaded uppers).

Insert the thumb screw into the rear set screw hole. Do not tighten.

Step U5.5: Align Gas Port and Gas Block

If using a borescope and gas system illuminator, feed the fiber optic cable down the gas tube from within the upper receiver and turn on. Insert the borescope into the barrel through the muzzle until the gas port is visible. Ensure the gas block is aligned with the gas port.

If using the gas block genie, seat and index the gas block such that the mark on the gas block aligns with the mark on the barrel.

Tighten the thumb screw when the gas block is properly aligned.

Step U5.6: Install Front Set Screw

Apply a drop of high temp thread locker onto the threads of one set screw.

Begin to thread the set screw into the front set screw hole in the gas block.

Using an appropriate hex wrench, hand tighten the front gas block set screw.

Step U5.7: Torque the Front Gas Block Set Screw

Set your torque driver to 25 in. lbs. and insert the appropriate driver bit (usually 3/32″) for your gas block’s set screws. Torque the set screws until the torque limit is reached.

Step U5.8: Install and Torque Rear Set Screw

Remove the thumb screw from the gas block.

Repeat steps U5.6 and U5.7 for the rear set screw.

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