How Does the AR Gas System Work?
Last Updated :: 12/14/2006 9:49:44 AM EDT
Randall's description of AR gas operation and how everything works in harmony- By Randall Rausch of

I have not written this up in a while and some day I really need to build a whole page dedicated to it.
This is just off the top of my head.
Much of it comes from Rick McDowel (competetion specialties) when I was first learning AR's & from Tweak along the way and my own experiences mixed in along the path to enlightenment.

Ok, starting with a cartridge in the chamber, hammer back.
Trigger lets the hammer fall.
Hammer hits the firing pin, driving it forward.
Firing pin drives the primer (and attached cartridge case) forwards in the chamber until the shoulder in the chamber stops the shoulder on the cartridge case.
The case will already be seated against the shoulder due to ejector tension, but the primer can sometimes move before the anvil legs on the primer stop against the primer pocket.

Headspace is the distance from the bolt face to the head of the cartridge when fully seated in the chamber.
Headspace gauges account for the length of the cartridge AND for the recommended amount of headspace, but what really matters is the amount of space, or lack there-of, of space between the case head and bolt face.

The firing pin continues forward to ignite the primer.
Primer flash ignites powder charge, instantly creating great pressure within the cartridge case.
Cartridge case expands first outward towards chamber walls (path of least resistance) where pressure holds the case in place and then the case stretches backward until the case head is stopped against the bolt face.
Here is WHY long headspace makes cases fail!
Bullet begins movement down the barrel, first encountering the throat.
Here is why you want a throat DIAMETER closely matching the bullet.
More about throat dimensions can be found here:
Loose throats do not control the bullet and keep it as straight while engraving into the rifling.
Now the bullet has obturated and engraved into the rifling and it's accelerating rapidly down the bore.
As it passes the gas port, gas begins to flow into the gas block where it turns and heads towards the bolt carrier via the gas tube.
The pressure is still high in the barrel, usually 15,00PSI+ until the bullet leaves the muzzle.
Just as the bullet leaves the muzzle, gas escapes around the base of the bullet.
Here is why a proper crown is important.
Gas is traveling about 5x faster than the bullet when it leaves the muzzle.
An even crown releases gas all the way around the bullet at one time.
An un-even crown lets gas go on one side first.
This can tip the bullet just slightly sideways at the moment the bullet is released into the air.
This is a very important time in the bullet's flight.
Now, remember, high pressure gas always follows the path of least resistance, which is now out the front of the barrel instead of into the gas system.
Barrel pressure drops immediatly.
During the bullet's travel down the bore between the gas port and the muzzle, we had a metered amount of gas fed to the action.
This gas does the following:
Upon reaching the gas key bolted to the top of the carrier, it turns down into the bolt carrier where it is given a nice place to expand.
This is the area inside the bolt carrier where the bolt lives.
Gas expanding here forces the bolt carrier back AND the bolt forward.
Note that the bolt is also being forced BACK by the gas pressure expanding the cartridge case on the other side of the bolt.
For a short moment in time, these forces are about equal.
Ideally, this is while the bolt lugs are unlocking and before the extractor starts pulling on the case.
The bolt carrier starts to move backwards against the inertia of the carrier's weight, the buffer's weight and the operating spring.
All of these effect timing, that's why we have different weights of carriers, standard, heavy (H), H2, H3 etc.
The next thing the carrier encounters are the cam surfaces against the cam pin.
Of course we know that the cam pin goes through the bolt.
Rearward movement of the bolt carrier causes the bolt to rotate.
(pay attention here, this is the meaty part)
Here is where timing comes into play.
Let's make a couple assumptions here before we continue.
Trust me that pressures in the case hold the case into the chamber, even though the chamber is slightly tapered.
Also trust me that when you release all the pressure out the front of the barrel that the cartridge case will spring back down to size so it's no longer a tight fit in the chamber as it was with the gas pressure present.
Here's where timing comes into play.
We want the bullet to be out of the front of the barrel AND the pressure to have subsided enough that the case shrinks down BEFORE the bolt lugs are unlocked because when the pressure is high, the case WILL try to stay in the chamber.
Now is the perfect time to point out that one sure sign of high pressures are the fact that the case extrudes into the ejector plunger hole on the bolt and the resulting pressure unlocks the bolt while pressures are still high.
This extruded brass gets wiped off the end of the case head, leaving a shiney spot and the brass usually makes it's way under the extractor, later causing extraction problems we will get to in a little bit.

Now back to extraction, normal/correct version:
Pressure subsides, bolt unlocks, carrier momentum continues rearward, pulling the fired (and contracted) cartridge case from the chamber.
As the cartridge case reaches the ejection port, the case pivots on the extractor hook from pressure of the ejector until it is sent flying free of the rifle.
The bolt carrier continues backward while re-cocking the hammer until operating spring pressure or the buffer stops it.
Operating spring returns the bolt carrier forward where it strips another round from the magazine up the feedramps and into the chamber.
Cartridge stops in the chamber, bolt continues forward, causing the extractor to snap over the rim of the cartridge case.
Bolt finally stops against the case head, but the carrier continues forward.
The cam surfaces in the carrier now cause the bolt to lock into battery again.
Now we are back where we started.

Now for extraction, the WRONG ways.
First, too much gas(most common):
The bullet has not left the barrel yet, but it's past the gas port.
Too much high pressure gas is rushing into the carrier, causing it to move rearward faster then desired and unlock the bolt from the extension.
Pressures are still high so the cartridge case is NOT ready to be extracted yet.
The carrier's momentum continues to pull backward, but the pressures in the case actually hold in in the chamber.
This causes a hiccup in the carrier's momentum.
Depending on the severity of the timing, several things can occur:
#1 The (weak) extractor spring allows the extractor to jump over the rim of the cartridge and the bolt carrier continues rearward, grabbing the next round and causing the classic "fired case in chamber, live round behind it" FTE.
The brass shavings under the extractor usually contribute to this one as well.
#2 The extractor does NOT slip off the case, but keeps pulling.
The extractor is strong enough to RIP the rim right off the case.
Same result as above, but MORE brass shavings everywhere from ripping case rims off.
#3 The extractor does NOT slip off the case, but keeps pulling.
During this pulling, the bullet has JUST left the bore, pressures recede and the case shrinks down, allowing extraction.
The rest of the cycle goes as normal, but you have strong pull marks on the case.
Recoil will be higher than normal when the carrier is allowed to travel to the end of the buffer tube and bottom out swiftly against the end of the buffer tube.
In normal operation, the buffer just kisses the end of the tube.

Somewhere between here and the next section, we have proper operation.

Lastly, not enough gas(less common):
The bullet is out of the bore, pressure is subsided, case is extracted and on it's way to ejection.
Depending on the severity of the lack of gas, the bolt carrier may not even get the case out of the chamber before the operating spring returns it forward.
Adding more gas, the case just barely gets out of the ejection port, but the bolt grabs it on it's way forward, classic stovepipe.
Add more gas and the cartridge clears the action, but the bolt does NOT get far enough back to strip a round from the mag.
This is classic short stroking.
You have a single shot action which extracts and ejects, then closes on an empty chamber after you fire it.
In this condition, the bolt will also ride over an empty magazine and close on an empty chamber.
Add some more gas and you will reach the point where it feeds from the magazine and ALMOST works properly, but it still closes over an empty mag.
This is two things, first, poor mag springs are not pushing the follower up fast enough to catch the bolt and second, the bolt is not quite making it back far enough to catch on the magazine follower.
Add just a little more gas and you are back to proper function.

Now, take note, that a lack of gas in a rifle that was functioning fine before can be from several things:
Gas key screws poorly staked and they loosened up, allowing some gas to escape instead of doing it's job INSIDE the bolt carrier.
Gas ring gaps are aligned, gas rings missing or broken, allowing extra gas to flow past them.
Gas block/front sight base is loose, allowing gas to escape before it even gets down the gas tube.
Gas tube "mushroom" is severly worn, probably because it was not properly aligned with the gas key and gas is escaping there.

Please note that failure to extract/eject is a symptom of EITHER too much or too little function of the action.
FTE alone is not enough information to decide what to change to fix the problem.
You need to look for other signs such as the excessive recoil and case rim pulling of too much gas or the short stroking of too little gas.
Unfortunately, many guys who don't understand the magic above always ASSUME that they have too little gas.
What do they do?
They open up the gas port.
Following the examples above, you can see this only makes the problem worse.

Lesson to be learned:
Follow the published troubleshooting procedures.
They are written that way for a reason.

Whew, that was longer than I expected to write.
Hope it all sinks in and you can benefit from it.

Edited 2-3-06 to add more about gas port pressures:

We often hear about mid-length being smoother cycling or pistol being harsher cycling than the typical carbine length gas systems.
Below is a plot of a 223 load.
I have noted the locations of the various gas ports in blue.
You can plainly see what pressures are introduced into the gas systems when the bullet JUST passes the gas port.
This is the reason for the way the various gas system lengths function differently.
Projectile travel at the bottom assumes that the bullet starts out about 1.5" from the breech, so add 1.5" if you want to compare velocities at different lengths.

Originally Posted By FredMan:
Maybe I'm wrong, but it's my understanding that headspace is the distance from the chamber shoulder to the bolt face; that a properly headspaced rifle will have little or no "slop" of the cartridge in the chamber. IOW, the cartdridge shouldn't move forward under the influence of the firing pin striking the primer because the cartridge is already fully seated, shoulder to bolt face, in the chamber.

Your understanding that there is zero clearance between the bolt face and cartridge case is incorrect.

Headspace is the amount of space between the cartridge case head and the bolt face.
The dimensions of the cartridge have LOTS of influence on the final headspace.
Because the dimensions of ammo are not consistent, steel gauges were developed to serve as a standard that can be repeated by multiple manufacturers.
There are established dimensions within the industry that specify how long a cartridge OR chamber should be.
These dimensions allow for manufacturing tolerances, so they are not an ABSOLUTE VALUE.
In the case of a chamber, we have headspace gauges to see that we are within spec.
The actual dimension of the chamber and ammo is NOT important as long as the two work together correctly.
You want to avoid more than about 0.005" headspace with your ammo/chamber combination.
For reliability's sake, you don't want to go much under about 0.002"

In the case of the AR-15 with it's bolt mounted ejector, the case is already being pushed forward and all the headspace will appear at the case head.
In something like an uzi or mauser, with a fixed ejector, the cartridge case does get pushed forward in the chamber as it's being fired.
There's actually a lot more going in in the chamber when the powder charge goes off, my version above was relatively simplified.
When the primer ignites the powder and the cartridge is already against the chamber shoulder, there is headspace at the case head.
Pressure actually pushes the primer back against the bolt face.
As the pressure gets higher, the whole case yields and stretches just in front of the case head to fill the chamber.
The reason that high pressure loads show flattened primers is that when the primer is hanging out the end of the case, it slightly balloons out and then gets sized back down when the case head slides back.
This is where the squared off primer shape comes from.
Read your PO Ackley and Julian Hatcher.
Lot's of good stuff on what's going on inside the chamber...

The bolt carrier starts to move backwards against the inertia of the carrier's weight, the buffer's weight and the operating spring.
My understanding here is that the gas escapes from the gas tube, through the carrier key, and impacts both the "back" of the bolt face and the rings on the tail of the bolt. The only real gas effect on the carrier here appears to be radial, not longitudinal. The bolt can't travel forward (locked against the barrel extension), so it travels backwards, operating the cam, unlocking the lugs, and pulling the carrier along with it. The carrier really is just along for the ride on the extraction phase of the firing cycle; it's only real contribution (aside from structural) is to provide a camming surface to alow the bolt to unlock. Oh, and I guess it does keep the bolt travelling in a straight line :).

You have a couple real important concepts backwards and I have underlined the important parts.
When in battery (locked) the bolt is captured both forwards (by the barrel/cartridge) and rearward (by the lugs on the barrel extension)
It is the carrier that moves rearward and cams the bolt via the cam pin to make the bolt rotate 22.5 degrees to unlock from the barrel extension.
By this time, the carrier's (and buffer's) inertia keeps it moving rearward and PULLS the bolt along with it via the cam pin.
How would the bolt travel backwards and react with the cam cut in the carrier if it's already captured by the barrel extension?
Does this make sense?

The gas does push forward against the bolt and rearward against the chamber within the carrier.
You are correct that the bolt has no way to move forward, so only the carrier can move and only rearward as it is at rest against the extension when in battery.
The back of the bolt and the gas rings are just there to seal the hole in the front end of the carrier.
The carrier is the piece that the direct gas impingement is having the effect on.
The INERTIA of the carrier and buffer are what tries to hold it in place AS WELL AS THE SPRING.
When pressure gets high enough (quickly) it overrides the inertia AND the spring and moves the carrier rearward.
It's the bolt that is just along for the ride during this.

Originally Posted By FredMan:
As for the gas impingment, it seems you are saying that carrier rearward movement is begun to be driven by the effect of gas on the carrier key,

No, the key just gets the gas inside the carrier.

even before the effect of gas on the bolt rings.

The gas rings on the bolt are important.
They confine the gas from moving forwards up along the bolt.
This way, the carrier takes all the energy that the expansion of the gas carries.

I guess the question that remains for me is this: does bolt unlocking occur primarily as a function of the carrier "pulling" the bolt rearward, or as a function of the bolt "pushing" the carrier rearward?

Neither, the gas pushes against the gas rings, the bolt and the carrier.
The bolt and rings are mechanically confined by locking lug alignment, the carrier is not confined except by ineretia and a spring, therefore the carrier moves back because of gas expansion inside.
As the carrier slides back, the cam pin is forced over in the cam pin slot and this causes the bolt to rotate.
It is the cam pin that unlocks the bolt.
After the cam pin hits the end of it's track, the bolt is already unlocked and it simply gets PULLED (along with the spent case) away from the chamber.

[ETA that upon further thought it has to be the carrier that performs primary unlocking function; as you said the bolt is locked fore and after by barrel extension and locking lugs. Only way for bolt to unlock is to rotate via the cam pin, and the only way for that to happen is for the carrier to do the "primary" moving]


To carry this thought further, what would be the effect of gas being vented from the carrier key to a point outside the carrier as opposed to inside the carrier?

The carrier would not be forced rearward without the sealing of the gas rings to create something to push against.
The gas does indeed get vented out of the carrier through the two exhaust ports.
Of course this does not occur until the gas rings pass the ports.
By this time, there is sufficient inertia to finish the cycle.

Originally Posted By CCW:
Can the trigger fall against the firing pin if the bolt is full forward in the chamber, against the brass, but not rotated and locked, or just partially rotated, ie. bolt carrier slightly back off of the full forward position? If not, why not?

The carrier blocks forward firing pin travel.
If the bolt is partially locked, the bolt carrier is still at least 0.075" from the barrel extension.
As the firing pin protrusion is somewhere around 0.040" when seated, the firing pin would not be able to reach the primer when being held back 0.075" by the carrier.
What would happen is that the extra energy of the hammer would tend to drive the carrier closed.
Maybe, BIG MAYBE, there is enough momentum to fire the primer, but not likely.

If the carrier is in motion, it will have finished locking before the hammer gets there if the hammer is released while the carrier is still back 0.0.075".
When you check the autosear timing on an M16, you are looking at the distance the carrier is from the bolt carrier when the hammer is released.
Off the top of my head, this is about 0.100"or less.
Any more and the hammer will beat the carrier and you get misfires.

I suppose another way to think of the head space clearance is that if you did not have clearance, you would have line to line contact between the bolt face and the brass or an interference fit between the same. The clearance allows an imperfect fit to still work over temperature ranges, over machining and assembly tolerance stack-ups, etc.

Exactly right, that's why we NEED some headspace, to allow for manufacturing tolerances.
Standards and Gauges are just an implementation method so that multiple makers in different areas can make products that are all compatible.

Originally Posted By Walmart_Special:
If the chamber pressure is high at beginning of the barrel and drops off as it travels down the barrel, then when the gas port is closer to the chamber, why do we need larger port size? Wouldn't the higher pressure provide more cycling force? If we need to match the pressure to longer barrel, won't we be using smaller hole?

It is also demostrated that short barrel can cause pre-mature extraction (high chamber pressure and with larger port size), people then use heavier buffer, longer gas tube to retard the timing. Why not just reduce the port size and reduce the pressure? (of course smaller port probably won't cycle)

Could it be that we need not only pressure but volumn (mass) for the gas as well?

HaHa, he gets it...

You need a certain VOLUME of gas to function the action, not a certain pressure.
You can get this volume with a large port and short duration or low pressure or with a small port and long duration or high pressure.

With the SAME barrel length, you make the port smaller as it gets closer to the chamber.
What you are doing here is two different things, first tapping into a higher PRESSURE gas supply, but ALSO increasing the DWELL TIME.
Now, when you shorten a barrel, while keeping the same gas system, you are simply reducing the dwell time and it's appropriate to enlarge the gas port accordingly.
If you are COMPARING two different barrels and both have the same amount of barrel length past the gas port, then the port will likely be smaller on the shorter barrel as that port is nearer to the chamber and therefore gets a higher pressure gas supply.

Running a proper size gas port or adjustable gas system is FAR BETTER than resorting to a heavy buffer (more reciprocating mass means more muzzle rise), but it's easier to just buy a heavy buffer and swap out parts than properly correct the gas flow.
Adjustable gas tubes are $60 and offer one more thing to go wrong and heavy buffers are like $15, what would you choose?
MOST AR's come over-gassed from the factory so that they will run correctly right out of the box without waiting for the gas rings to seat into the carrier.
Once the gas rings seat, the rifles are more likely to show signs of over gassing.
That's one more reason heavy buffers and o-rings on extractors are so commonplace.
For these very same reasons, you can take a 16" (carbine length gas system) barrel, cut it to 14.5" and they run perfect.
This is not always the case though, I recently cut an LMT MRP 16" (mid-length gas system) down to 14.5" which DID require being opened up some.
I was pleased that the MRP barrel gas port was more towards the small side of the range.