How to Build a Gas System Illuminator

Correct gas block alignment is critical to efficient and reliable gas-system operation. The most direct way to verify alignment is to view the barrel gas port from inside the bore while illuminating the gas path through the gas tube.

This requires two tools:

1. A borescope inserted through the muzzle to view the barrel gas port from inside the bore.
2. A gas system illuminator inserted into the gas tube from the receiver end of the upper.

The illuminator uses a side-glow fiber optic cable to introduce light into the gas tube. That light passes through the gas tube port, the internal passage in the gas block, and the barrel gas port. When viewed through the borescope, the illuminated opening shows whether the gas block is properly aligned with the barrel.

The tool is simple and inexpensive to assemble using commonly available components.

The inspiration for this tool is available for sale, fully assembled, from Xtreme Effects.

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Components:

You will need the following components.  All can be purchased from Amazon.  It will cost about $25-30 per illuminator (you will spend around $40-45 total because some components come in multi-packs).

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Tools:

The following tools are required for the barrel connector version only.

• Phillips-Head Screwdriver
• Wire Cutters and Wire Strippers
• Heat Gun (optional)
• Soldering Iron (optional)
• Solder (optional)

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Assembly Instructions:

The following procedure describes how we assemble the illuminator. Other secure electrical-connection methods may also be used.

Pre-Assembly (Barrel Connector Version Only):

1. Remove the original connector.
   Cut the snap connector from the fiber optic illuminator.
2. Prepare the illuminator wires.
   Separate the two wires and strip approximately 3/4 inch of insulation from the end of each wire.
3. Prepare the barrel-connector wires.
   Separate the two wires on the female barrel-connector pigtail and strip approximately 3/4 inch of insulation from each end.
4. Position the heat-shrink tubing.
   Before joining the wires, slide a 1.5-inch length of 1/8-inch heat-shrink tubing over each barrel-connector wire. Then slide a 1.5-inch length of 3/16-inch heat-shrink tubing over both wires (this larger piece will later reinforce the completed connection). Move the tubing far enough away from the exposed wire ends that it will not begin shrinking while the connections are soldered.
5. Join the corresponding wires.
   Twist the matching wires together, maintaining the correct polarity — typically red to red and black to black.
6. Solder the connections.
   Solder each twisted connection and allow it to cool completely.
7. Insulate and reinforce the connections.
   Slide the smaller heat-shrink tubing over each soldered connection. Apply heat until the tubing fits tightly around the splice. Then slide the larger piece of heat-shrink tubing over both individually insulated connections and shrink it into place. This outer layer helps support the wires and reduces stress on the splices.

Final Assembly:

1. Install the fiber optic cable.
   Loosen the plastic retaining screw on the fiber optic illuminator using a Phillips-head screwdriver. Insert approximately 18 inches of 2.0 mm side-glow fiber optic cable into the illuminator’s output opening. Tighten the retaining screw only enough to hold the cable securely. Do not crush or deform the cable.
2. Connect the dimmer switch.
   Plug the illuminator into the rotary dimmer switch.
3. Connect the power supply.
   Plug the rotary dimmer switch into the power supply.

The gas system illuminator is now ready for use.

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Instructions for Use:

Before beginning, verify that the firearm is unloaded and that no ammunition is present in the work area.

1. Separate the upper receiver from the lower receiver.
2. Remove the bolt carrier group and charging handle from the upper receiver.
3. From inside the upper receiver, carefully feed the fiber optic cable into the open rear end of the gas tube. Advance it toward the gas block as far as it will travel without forcing it.
4. Insert the borescope through the muzzle and position its side-view mirror so that the barrel gas port is visible from inside the bore.
5. Plug in the power supply and turn on the illuminator using the rotary dimmer switch. Begin at a low setting and increase the brightness as needed to obtain a clear image.
6. Observe the barrel gas port through the borescope while adjusting the position of the gas block.

7. Once alignment has been verified, secure the gas block in place, turn off the illuminator, remove the fiber optic cable and borescope, and complete the gas block installation.

For the complete gas block installation and alignment procedure, see Step U4: Install the Gas System.