Understanding the AR-15 Gas System

The AR-15 Direct Gas Impingement System is a rather straight forward system to understand. When the weapon is fired, gas travels down the barrel behind the bullet. As it passes the gas port, it is diverted into the Front Sight Base (FSB) or Gas Block into a gas tube which carries it into the bolt carrier key. Once
the gas arrives at the key, it dumps the gas into a chamber causing the bolt carrier to move rearward.

                                             (Elements of the gas system)

Gassing starts in the chamber with the round of ammunition. This means that ammunition selection is important. Ammunition may be loaded to either .223 specs or 5.56 NATO specs. Ammunition produced with poor quality control can affect the operation of the weapon.

From here we move to the gas port. The gas port is very important to the overall function of the rifle. In the military world this is virtually a non-issue since gas ports are standardized. In the commercial/civilian world a manufacturer can gas their barrel anyway they want. One of the first things I do when I assemble the upper or conduct troubleshooting is to measure the port size. If one knows the correct sizing, this can save a lot of trouble in the process.

(Using a pin gage to get correct port size)

The most solid gas block a person can have is the pinned front sight base (FSB) which is found on almost all military styled M16’s/M4’s and variants. The FSB acts as the gas block and front sight.

(MILSPEC F-Marked FSB pinned to barrel)

With the proliferation of rail systems the need came about for low-profile gas blocks (LPGB). Unfortunately these modifications and the proliferation of the AR into the commercial (non-military standard) environment have resulted in issues. The use of substandard components combined with sloppy craftsmanship can result in a compromise of the gas system (leakage) which causes malfunctions.
One such substandard component is the aluminum gas block. Many of the bottom tier manufacturer’s will use these because they are very cheap. The aluminum gas block suffers from two major issues. The first is thermal expansion and the second is erosion of the gas block from the hot gases.

                                          (Aluminum gas block erosion)

These two issues create leakage within the gas system which result in the malfunction known as “short-stroking”. Most often when these gas blocks are installed onto the barrel, the manufacturer will also not dimple the barrel which results in gas block shift. This too will cause a malfunction at some point.
Moving onto properly made steel type gas blocks we can see that there are differences as well. Not only in the overall length and design of the block, but also the spacing of the holes for the set screws. This is important to know and it is one of the reasons why most manufacturers that use such blocks will only dimple the barrel below the set screw hole below the gas port.

(L-R: Unknown block, BCM block, Geissele SGB, Centurion Arms MK12 block)

This in turn leads to another issue with the use of low-profile type gas blocks and that is the correct installation. Of prime importance is that when the gas block is installed, the gas block does not obstruct the gas port. Most all properly made barrels will have a gas port which is approx.  .295 inches from the shoulder to the gas port center. There will most always be a small allowable tolerance here due to the size of the gas port orifice. In general terms we want the tightest fit we can get between the barrel and gas block.

(Gas port location)

In regards to the gas port orifice I have generally seen that the size of the port is usually about .154-.160 inches in diameter across a wide variety of manufacturers. This means that in many cases the gas port orifice is about 2-2.5 times larger than the gas port itself.

(Centurion Arms MK12 orifice size .158 inches)

Now that we have covered the gas port, FSB, and gas blocks let’s move onto the gas tube. Generally when we install a gas tube into the FSB or gas block, we want a good snug fit. Out of spec tubes, or a sloppy fit can result in gas leakage. It’s important to note there is almost always a small amount of gas leakage. Usually a small amount of leakage isn’t enough to stop the cycle. But, if we couple gas leakage at the block, with an out of spec gas port, and some other issue (like bad gas rings) then we have a recipe for disaster.

(Example of typical gas leakage)

The next area of concern for the gas tube, is the where the gas tube meets with the bolt carrier key. A correctly made gas tube will be flanged at the top where it meets the key and provides a seal. If this area wears down, then gas will escape. The cause of excessive wear is a misalignment of the gas tube with the carrier key. This is also directly related to proper installation to include gas block alignment.

(New gas tube and bolt carrier key orifice)

(Top- Worn gas tube. Bottom new gas tube)

Well now comes the pièce de résistance, the Bolt Carrier Group or BCG. This subject is not without controversy. There are but a handful of companies/manufacturer’s that get this right. In 30 years of working with the AR-15 family of weapons I have seen so many bad staking jobs that it’s not funny.

So let’s discuss a few facts. Stakings have changed over the years. Original carriers which were hard chromed had a different staking which was done with a center punch at the top of the carrier. This later changed to (2) staking points on each screw. Additionally as referenced in an old print, Permatex #3D was applied in an area below the carrier key. I have not seen this done in several years and I cannot be certain it is still a requirement.

(Original Colt Model 601 Bolt Carrier)

The torque value of the carrier key screws is very minimal (58 in./lbs.) which makes the staking even that much more important. Equally as important are the correct screws. Here’s a hint. If you see YFS on the head of the screw, it’s not correct.

(MILSPEC screws with correct staking)

There are those who refuse to believe that these screws can and will loosen over time with use. I have seen it so many times, I have lost count. Sadly, many cannot do it correctly. If the carrier key is not seated correctly, the screws are not torqued and staked properly they can and will loosen. This results in massive gas leakage as well as damage to the gas tube.

(Incorrect carrier key stakings)

(Incorrect screws and carrier key stakings)

And last but not least, we have the bolt gas rings. The gas rings are essentially the cheapest part of the gas system, and probably the most overlooked. The average price of a set of gas rings (they come in 3’s) is less than the price of a Venti mochacchino caramel drizzled Latte.

Gas rings)

A question which often times comes up is “How do you know if they are bad”? There are two tests that are used. The simplest and most common is as follows; with a clean bolt and bolt carrier, insert the bolt into the carrier opening. Extend the bolt into the unlocked position (without the cam pin) and stand it on its face. If the carrier collapses onto the bolt under its own weight, then you should replace your rings. The standard rule is you replace all three at the same time. Another quick note on gas rings. The myth that gas rings must be staggered to prevent the splits from aligning and causing gas leakage, is just that- a MYTH. There is no reason to do so.

                                                      (Gas Ring Test)

I hope that all AR-15 owners and users will find this information beneficial when making a future purchase of a weapon, parts or performing maintenance.

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William Larson
Owner/Instructor at Semper Paratus Arms, LLC
Will Larson is a veteran of the U.S. Army Infantry and the U.S. Coast Guard where he served as a Gunner's Mate. After leaving the military with almost 10 years of military service he entered into the world of contracting working primarily in Iraq and Afghanistan for 6 years. He is the owner/instructor for Semper Paratus Arms, LLC which teaches AR-15 based armorer and builder courses and he is the master armorer for SIONICS Weapon Systems in Tucson, Arizona. He has completed numerous factory and field armorer courses and a variety shooting courses and has close to 30 years of experience with the AR-15 family of weapons.

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