How AR gas operation works by Randall_Rausch
Much of it comes from Rick McDowel (competetion specialties).
Starting with cartridge in the chamber, hammer back.
Trigger releases hammer which 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 firing pin continues forward to ignite the primer.
Primer flash ignites powder charge, creating great pressure in the cartridge case. 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.
Bullet begins movement down the barrel, first encountering the throat.
Here is why you want a throat DIAMETER closely matching the bullet.
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 flows 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, until the bullet leaves the muzzle.
Just as the bullet leaves the muzzle, gas escapes around the base of the bullet. A good crown releases gas all the way around the bullet at one time.
A bad un-even crown lets gas go on one side first which tips the bullet just slightly sideways at the moment the bullet is released into the air. Now, remember, high pressure gas always follows the path of least resistance, which is out front of the barrel and 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.
Upon reaching the gas key, it turned 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. Expanding Gases forces the bolt carrier back AND bolt forward.
Note 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 Balanced and this is when the bolt lugs are unlocking and before 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 forces effect timing, that's why we have different buffers, springs etc.
Next the carrier encounters the cam surfaces against the cam pin which causes the bolt to rotate.
a couple assumptions, pressures in the case hold the case into the chamber,
when you release all the pressure the cartridge case will spring back down to size so it's no longer held in the chamber.
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 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.
Now back to extraction, normal/correct version:
Pressure subsides, bolt unlocks, carrier momentum continues rearward, pulling the fired 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.
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, bolt continues forward, causing the extractor to snap over the rim. 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. Finis
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, depending on 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.
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, so they open up the gas port only to make the problem worse.
Lesson to be learned:
Follow the published troubleshooting procedures, as written that way for a reason.
Below is a plot of a 223 load.
You can plainly see what pressures are introduced into the gas systems when the bullet JUST passes the gas port.
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.
Ok, what you have here is actually two charts that were put together.
On the RED Chamber pressure line chart you also see Blue marks representing the area on the pressure line where the bullet would pass the gas port and gas would be sent back down the gas tube to the chamber to move the carrier and unlock the bolt.
You'll notice that the further away the port is from the chamber the lower the chamber pressure drops so carbine is above 30,000 but mid-length is below and rifle is lower than 20,000. Remember the original barrels were all 20 inch.
The BLACK line traces bullets velocity as messured at the muzzle. here you have to add 1.5 inches to the measurement at the bottom of the chart.
Linear Mode
