Troubleshooting the Gas Gun

(This is a piece I wrote some years ago for a shooting forum.)

Since many forum members are shooting gas operated rifles and several of you have had problems, I thought I might describe the mechanical processes involved in one firing cycle. By understanding the mechanics involved, it may make it easier for you to determine where a problem is occurring and what the cause might be. I’m going to describe a gas operated, closed bolt, semi-auto rifle typified by the FAL, AR, AK, Garand, etc.

There are varying degrees of probability that any problem will occur and varying degrees of difficulty in correcting the problem. Some problems may be corrected with a few moments and no tools. Other problems may require a machine shop and be easier or cheaper to fix by replacing major components.

Before we start any gunsmithing, there are a few principles that we need to apply. The first principle is similar to the doctor’s oath- do no harm. Don’t make a problem worse than it already is and don’t make new problems. If you are uncertain about what you are doing, consider having the work done by a professional.

The next principle is to be certain what the problem is. An incorrect diagnosis (again with the doctors) will waste time and effort and may cause more serious problems (back to principle one). Quite often I read someone’s description of a problem and immediately consider more than one possible cause. A series of questions or an examination of the weapon is necessary to determine the exact cause. Meanwhile, several well-meaning individuals will respond with fixes for the most common cause or the problem they have personally experienced; whether these fixes are correct or not. Beware someone who states with absolute authority what the problem is without ever examining the weapon.

The third principle is to use the correct tools. It’s almost always cheaper to buy, borrow or rent the correct tool than to have to replace a piece damaged with an incorrect or jury-rigged tool. Gunsmithing screwdrivers are different from standard, household screwdrivers. If you don’t know how to grind your own screwdrivers, invest in a good set. An addendum to this third principle would be to know how to use the tools correctly.

The fourth principle is to try to work on the least expensive part. It’s cheaper to start filing on a front sight housing and wreck it than it is to start filing on the barrel and wreck that. Sometimes it’s not possible to correct the problem without modification or replacement of a major component. Remember that any work you do will probably void the warranty. Make certain that you don’t want to try to get repairs done under warranty before you begin work. Most manufacturers will warranty incorrectly manufactured parts as long as they’re still in business.

Working on the least expensive part may mean expense in terms of time rather than money. Let’s say you have a problem with two possible causes. The first is the most probable but will involve major machine work to correct. The second is much less likely but will be a quick and easy fix. Despite the lower probability, it would be advisable to try the quick-fix first rather than spend the time and effort on the major fix only to find out it wasn’t the problem. Otherwise, it’s generally best to start with the most likely problem and work your way down.

Fifth, don’t use force. If force is necessary, make sure you apply it correctly.

A quick word on military manuals; remember the old saying, "There’s the right way, the wrong way, and the Army way." The manuals were not handed down on tablets from the Mount. Military manuals may have errors but, more importantly, they will present one lowest-common-denominator method for repair. Plus, with Uncle Sugar paying the bills, replacing damaged bits is less of a concern. Often, there is an easier or better method for doing things. Just keep an open mind.

The operating cycle is Feed, Fire, Extract, Eject and back to Feed. By isolating which action in the cycle is failing, you’ve immediately narrowed the possible causes considerably. I’m going to start the description with the Fire portion of the cycle.

A round is already chambered, the safety (or safeties) is disengaged, and you’re about to pull the trigger. The trigger is pulled back and actuates the sear mechanism. The sear releases the hammer and the firing pin is forced into the back of the cartridge. Headspace or the extractor holds the cartridge firmly against the bolt face so that the firing pin can strike the primer with sufficient force. The firing pin penetrates the primer deeply enough to crush the priming pellet. The priming pellet ignites the powder charge which burns and turns into a high-pressure gas. The gas expands the cartridge case to seal the rear of the chamber and grip the sides of the chamber. The bullet is pushed down the barrel by the gas. As the bullet passes the gas port, gas begins to bleed through the gas port and into the gas system. The gas presses against the gas piston (or in the case of the AR, the carrier key) with sufficient force and volume and over a sufficient period of time to overcome the recoil spring, the mass of the reciprocating parts, and the mechanical friction, and start the bolt carrier moving back. As the bullet leaves the barrel, the gas pressure drops so that it is safe to open the action. The bolt carrier cams the bolt out of lock and the bolt begins to move back.

The extractor is already engaged with the rim of the cartridge. As the bolt moves back, the extractor pulls back on the empty case with sufficient force to overcome the friction between the expanded case and the chamber walls. The extractor will continue to pull back on the case until ejection occurs.

The two basic types of ejector are the bump type (AR) and the fixed type (FAL). The bump ejector is a little spring loaded plunger in the face of the bolt. The bump ejector is applying pressure against the base of the cartridge as long as it is against the bolt face. The fixed ejector is generally a blade attached to the receiver that passes through a slot in the bolt as the bolt moves back. The fixed ejector doesn’t contact the cartridge until the bolt moves sufficiently rearward.

As the bolt moves back, taking the extracted cartridge with it via the extractor, it reaches a point where the fired case will clear the ejection port. At this point, the bump ejector is pushing to pivot the case around the extractor. As soon as the case clears the ejection port, the bump ejector will throw it clear of the weapon.

When the bolt moves back to the point where the fixed ejector will strike the base of the case, it too will pivot the case around the extractor and eject it clear of the weapon.

The bolt carrier will move as far rearward as the gas pressure/volume allows. As it moves, it will push the hammer back until it clears the sear and disconnector. The bolt carrier will compress the recoil spring.

The top round in the magazine is held down against the magazine spring pressure by the bottom of the bolt carrier. When the carrier moves back far enough, the follower is pushed up by the magazine spring and forces the top cartridge against the feed lips. The recoil spring begins to push the bolt forward. The edge of the bolt face hits the base of the new cartridge and begins to move it forward. The nose of the bullet or the shoulder of the cartridge hits the feed ramp and tips up toward the chamber. The cartridge moves forward far enough to clear the front of the feed lips on the magazine and pops fully in line with the bolt and chamber. As the bolt carrier clears the hammer, the hammer begins to move forward where it is caught by the disconnector. While the trigger is held back, the disconnector will continue to hold the hammer back.

Note that incorrect headspace can cause problems with the next few actions. The shoulder of the cartridge will strike the front of the chamber and stop. The bolt will continue to move forward and the extractor will snap over the cartridge rim where it will be held by pressure from the extractor spring. The bump ejector will be compressed against its spring at this time. When the bolt face hits the base of the cartridge the bolt will cease its forward motion. The bolt carrier will continue forward and cam the action into lock. The forward motion of the carrier will cease.

When the trigger is released, the hammer will fall from the disconnector and be caught by the sear. The rifle is ready to fire again.

The idea when you have a problem is to compare what was supposed to happen with what really happened. By looking at the mechanisms and movements involved, you can determine a possible cause or causes. Then, you can test the potential causes until you identify and correct the right one.

There are all manner of gas and firing systems out there as well as recoil operated weapons that don’t use a gas system at all. Some of the systems can be real oddballs. The above describes what will be found in the typical AR/AK/FAL/Garand type rifles.

As I get time, I may try to describe the various specific problems that can occur and their possible causes/corrections.