Gas System Dimensions and Tolerance
What is tolerance?
We cover dimensions and tolerance in detail in our Spec to Inspect series about technical inspection and gauging of the AR-15. We strongly recommend that you read through those articles for a more thorough understanding of the principles we cover in the remaining articles in this series.
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.
Every standard component of the AR platform has a set of specifications per the Military Technical Data Package (TDP). This specification will include everything from dimensions, tolerance, treatments, finishes, and testing. For our discussion of gas system efficiency, we are particularly interested in the dimensions and tolerance.
If you have read through our Spec to Inspect article on Dimensional Specifications, you will recognize the following diagram, which illustrates the impact of dimensions and tolerance on the fitment of interfacing parts (a pipe and fitting).
This example is particularly relevant to the AR-15 gas system, as it is a fluid path that relies on tight junctions between interfacing components.
Why is tolerance important to the AR gas system?
When you stack loose tolerances along the gas system, you have leaks, leading to an inefficient gas system. The more inefficient the gas system is, the more gas you need to pump through the system to deliver the same amount of force at the BCG. A leaky system with excessive pressure loss is more sensitive (less robust) to variations in the inputs (powder charge, powder temperature, lubrication, soiling, etc.).
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 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.
What can I do about an inefficient 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 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.
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.
- 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. Per the previous article, our recommendation is to regulate the flow of gas with an adjustable gas block. This approach will ensure that your gun will 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.
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