Gas System Efficiency

What is Efficiency?

Efficiency basically expresses how much something can do with what it is given. Said more formally, efficiency is a ratio of the magnitude of an output to that of an input. As it pertains to the gas system, it tells us how much of the gas entering the system actually has an impact on the operating system. “Efficient” describes tight tolerances of the component interfaces in the gas system.

What is Tolerance?

Tolerance refers to an acceptable degree of deviation around a target value. Generally, there will be an upper limit (at or above the target) and a lower limit (at or below the target) indicated in a tolerance specification.

The dimensional specifications for the gas system are defined in the Colt Technical Data Package. Learn more about dimensions, tolerance, and the TDP in our Spec to Inspect Deep Dive series.

Why is tolerance important to the AR gas system?

Dimensional specifications and tolerance are particularly important to the gas system. This system relies on tight junctions between interfacing components. Loose tolerances result in excessive leaking, which reduces the efficiency of the system.

A less efficient gas system requires more gas flow and pressure to achieve the same endpoint.

A leaky system with excessive pressure loss is more sensitive (less robust) to variations in the inputs and external forces (powder charge, powder temperature, lubrication, soiling, etc.).

What is an "efficient" gas system?

No gas system is 100% efficient. A “perfect” system is estimated to have an efficiency of about 40% (i.e. for every 10,000 psi at the gas port, it will deliver 4,000 psi to the end of the line), and very few systems are that efficient. The amount of loss in the system is proportional to the tightness of the junctions along the fluid path.

System pressures are highest at the gas port and decline due to loss and restriction at each junction as the gas works its way rearward. So, a loose junction between the barrel and gas block will generally result in a greater loss of pressure than an equally loose junction within the BCG.

With these principles in mind, the following list describes the dimensions and junctions that affect the efficiency of the gas system.

Gas Port
- If the gas port is undersized, the flow of gas will be restricted, leading to low gas volume flowing through the gas system.
Gas Block Journal/Gas Block Junction
- If the gas block bore is oversized and/or the gas block journal is undersized, this junction will leak excessively.
Gas Block/Gas Tube Junction
- If the gas block outlet is oversized and/or the gas tube is undersized, this junction will leak excessively.
Gas Tube
- If the bore of the gas tube is too narrow (i.e. if the wall thickness is excessive or the overall diameter is undersized), the gas flow will be restricted.
Gas Tube/Gas Key Junction
- If the gas key bore is oversized and/or the gas tube is undersized, this junction will leak excessively.
Gas Key/Carrier Junction
- If the gas key is not sealed to the carrier with a gasket material (Mil-Spec calls for Permatex), this junction will leak excessively.
Carrier/Bolt/Gas Ring Junctions
- If the carrier bolt tail run is oversized and/or the bolt tail is undersized, this junction will leak excessively.
- If the carrier gas ring run is oversized and/or the gas rings are worn or undersized, this junction will leak excessively.

Any one or two of these issues will generally not stop an AR. The AR wants to run. But most commercial systems have issues at several of these junctions. The gun may still run, but will be far less robust to variation.

How do I deal with the efficiency of my gas system?

You can wing it and deal with the symptoms of whatever efficiency you have. This usually means accepting loss of reliability, consistency, robustness, longevity, and/or cleanliness.

If you have an under-gassed gun, you could resort to a lighter BCG, buffer, and/or buffer spring. However, even if you can get it to function properly, the gun will be much more sensitive to variations in temperature, soiling, lubrication, ammo, etc. and will be much less reliable. If you want to tackle the root cause, you need to focus on dimensions and tolerance.

If you have an over-gassed gun, you are in a much better place than someone with an under-gassed gun. Assuming the gun feeds properly, you could do nothing and accept the excessive recoil and damage to the components of the gun. You can use a full-mass BCG and experiment with various combinations of heavy buffers and heavy springs to manage the excessive kinetic energy until you get reliable ejection and feeding. If you have the luxury, you can install an adjustable gas block which will allow you to control how much gas is flowing through the system.

How do I build an efficient AR gas system?

If you want to build an efficient gas system, you need to ensure the tolerances of each mating component are compatible with its mate, if not conformant to the Military Specification (which ensures compatible tolerances by design).

You can do this verification yourself, but you will need to know the ideal dimensions for each component. You will need to get your hands on the corresponding high-precision pin gauges, ring gauges, calipers, depth gauges, and micrometers to measure everything properly. This adds up to a lot of money and the investment is not rational for most shooters. Even if you have the equipment, you’ll need to know what you are doing, so you will probably need some hands-on training (School of the American Rifle is top notch). Even if you have the equipment and know how to use it, inspection will only tell you that something is wrong. You will still need to do something about any issues identified. This means correcting issues via gunsmithing and/or replacement parts. If you are interested in going this route, please check out our Spec to Inspect series on technical inspection and gauging of the AR-15.
You can buy upper receiver assemblies and BCGs from a builder who understands tolerance and gauges components to ensure tight fit. Unfortunately, most builders and manufacturers don’t understand tolerance, and if they do, they don’t give it much attention (because it takes a lot of time and effort and consumers buy the products either way). Do not trust the “Mil-Spec” label on consumer components, assemblies, and firearms. Consumer manufacturers use it as a buzz word and there is no verification or official compliance monitoring of the consumer market.

If you build (or re-build) an efficient system, you likely have more gas than you need, so you will need to manage it as an over-gassed gun. Our recommendation is to regulate the flow of gas with an adjustable gas block. This approach will ensure that your gun can run under the best and worst of scenarios and can easily adapt to all conditions. If you go this route, you will have the most robust system available in an AR platform.

Inspecting the AR Gas System

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.

We will not get into the technical aspects of gauging, measuring, and inspection here. We cover this in great detail, including a review of the TDP and the associated specifications. Check out Spec to Inspect for a thorough review.

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.