Gauging and Inspecting the AR-15 Gas System

Inspection and testing are two major tenets of Quality Control. Please check out our Spec to Inspect series for additional information on inspection, including measuring and gauging the AR.

Types of Quality Control

Visual Inspection is exactly as it sounds: verification of the condition of a component using your eyes (aided or unaided). We use visual inspection to detect defects in finish and damage to components.

Technical Inspection involves an objective procedure to evaluate the condition of a component. Technical inspection can be both qualitative (e.g. pass/fail) or quantitative (e.g. a measured value). Technical inspection may require tools, instruments, or gauges. Gauging and measuring are two types of technical inspection that we use for Quality Control of the AR.

Testing involves an systematic verification of the correct function of a component or system. This can range from a pressure test to a live fire test. Testing is generally the “end-of-the-line” evaluation. In theory, if all of the components of a system meet specifications and are assembled properly, the system should function properly. Testing is that verification.

Tools for the Job

If you want to measure or gauge components of the gas system on your own, you will need a few things:

You need the dimensional specifications listed in the U.S. Military Technical Data Package (TDP) drawings for each component. We have made a pretty comprehensive collection available in our Spec to Inspect series.
You need to translate the TDP into meaningful specifications. We combine the TDP specifications with School of the American Rifle (SOTAR) specifications. SOTAR specifications are not openly distributed, so we will not share them. We share our PBA gauging and measuring specifications in our Spec to Inspect series.
You need the appropriate gauges and instruments, based on the relevant specifications. We cover these tools in great detail in our Spec to Inspect series. The relevant gauges and instruments for the gas system may include:
1. Pin Gauges: Pin gauges are used to check the inner diameter of a hole.
2. Ring Gauges: Ring gauges are used to check the outer diameter of a shaft.
3. Micrometer: Micrometers are high precision instruments that measure the width of an object.
You need to learn how to use your gauges and instruments. You will not find a better AR-15 armorer and gunsmithing course than the SOTAR Master Armorer Course. In this course, you will learn how to gauge everything worth gauging on the AR (among other things).

If you are considering measuring and gauging your gun, please remember that measuring and gauging only detect defects. They do not correct them. If you want to remedy any defects that measuring and gauging may identify, you may need gunsmithing skills and/or replacement parts.

What To Focus On For Efficiency

We’ve already discussed the gas system in great detail. By now, you should know what we are going to tell you to gauge before we tell you what to gauge. If you don’t, go back and read the previous articles.

Barrel

Gas Port: You want to make sure the gas port is sized appropriately for the gas system length.
Gas Block Journal: If you have a 0.750″ gas block journal, make sure its as close to 0.750″ as possible. You want about 1/1000″ play (combined) or less between gas block and gas block journal.

Gas Block

Main Bore: As with the gas block journal on the barrel, you want the main bore of the gas block to match as closely as possible. Again, you are shooting for 1/1000″ gap (combined tolerances).
Gas Tube Bore: You want the gas block to fit as tightly as possible around the gas tube. This is where you will lose the most initial port pressure.

Gas Tube

Front Diameter: You want the plugged end of the gas tube to fit as tightly into the gas block as possible.
Rear Diameter: You want the rear end of the gas tube to fit as tightly into the gas key as possible.

Gas Key

Gas Key Bore: You want the bore of the gas key to be ever-so-slightly larger than the gas tube ferrule. This is the insertion point of the gas tube and is another major point of loss of gas.
Gas Key Sealant: You want to make sure your gas key is sealed with a gasket material (e.g. Permatex or Loctite 620). If your gas key is not sealed, it is far more likely to leak. You can verify if the gas key is sealed by running a pressure test. We will post the pressure testing procedure soon.
Gas Key Screws: You want your gas key screws to be properly torqued and staked. If the gas key screws are (or become) loose, the gas key will lift off of the carrier and gas will escape between them. You can verify staking visually and perform a reverse torque test. Your target is 30 psi (screws should not move).
Gas Key Alignment: The gas key needs to be perfectly aligned with the longitudinal axis of the bolt carrier. The best tool for this step is a gas key staking jig.
Gas Key Bore Fluid Path: Sometimes, manufacturers get sloppy with gas key sealant. Sometimes they forget to drill the appropriate hole in the top of the carrier. The best way to verify that the fluid path is clear is with 0.080 string trimmer line.

Bolt Carrier

Carrier 3-Bore: There are 3 important bores in the carrier: the bolt shoulder run, gas ring run, and bolt tail run. The latter two are critical to the efficiency of the gas system.
Carrier 3-Bore Finish: The interior of the carrier 3-bore needs to be perfect. The finish should be hard chrome, DLC, or a comparable finish (nitride is not an acceptable finish on this surface, but is very common in the consumer BCG market). There should be no machining marks, no scratches, no incomplete coverage, etc.

Bolt Assembly

Bolt Shoulder Diameter: To adequately support the movement of the bolt in the carrier, the bolt shoulder must be very precisely matched to the carrier bolt shoulder run. This is not directly related to the gas system, but does support proper alignment of the bolt in the carrier.
Bolt Tail Diameter: The bolt tail must be small enough to fit through the carrier bolt tail run, but large enough to seal the piston chamber created inside of the BCG.
Gas Rings: The gas rings must be inspected regularly for wear. The rearmost ring will wear the fastest and the foremost will wear the slowest. The efficiency of the gas rings can be verified objectively by a gas ring efficiency test.

There are many (many) more important inspection points on the AR platform. But these are the ones most critical to gas system efficiency.