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Russ Larges Posted - January 02 2019 : 10:27:22 AM
I have often read of over or under gassing of the AR platform but really don't know much about it. I can see how under gassing can cause a malfunction and over gassing can beat up a gun. But how does one tell if the gun is over gassed. Now we come to adjustable gas systems and the options they offer, to make it worse we have different gas systems, pistol, carbine, rifle. Makes my head spin, can anyone clear up some of this for me.
Thanks Russ
14   L A T E S T    R E P L I E S    (Newest First)
LittleBill Posted - May 05 2019 : 10:00:00 AM
Great thread! Reads like a primer on how the AR system works, for those of us who weren’t familiar with the ins and outs.

Thanks, gents!

WR Moore Posted - May 04 2019 : 8:13:10 PM
Doing a little net surfing while the dogs eat and ran across this thread again. There is one fairly easy way to suggest serious over gassing. At the back of the bolt buffer is a truncated cone of some soft plastic. You can pick up a replacement item from Brownells for a couple bucks as an exemplar. If you check it periodically for battering and it's looking worse for wear in short order, it might be a good idea to experiment with a heavier buffer. They come in standard semi-auto carbine, H1, H2 (standard in M4s) and H3.

This assumes a proper recoil/bolt spring. The rifle spring should be 41-43 coils, minimum length of 12 in. The carbine spring should be 37-39 coils, 10 inch minimum. New springs are roughly 12 3/4 inch from rifle and 10 1/2 inch for carbines, but this can vary.
Jim Higginbotham Posted - January 14 2019 : 11:30:10 AM
I cannot add much to the excellent coverage above but would point out that port pressure depends a little on how long the bullet has until exits once it is past the port (actually that was covered above as dwell time).

With the same length gas tube in the AR, a 16" barrel with a carbine length tube, might be more "overgassed" than a pistol with an 11.5" barrel and the same length gas system.

If it is a .223 and a pistol length tube (much shorter than the carbine) then it is different.

Ten Driver has the straight scoop on how to adjust one.

Jim H.
gw Posted - January 13 2019 : 08:57:09 AM
an over gased gun will run dirty, excessive black soot

When an AR is over gassed bolt speed increases, rounds will not be picked up or the bolt will not lock back on an empty magazine. It can also effect how the gun ejects

the military controls the ammo they use, civilians can use any available, including the steel cased stuff.

for that reason, I'd assume manufactures build AR pistols overgassed, but it's a matter of degrees.

if the pistol runs ok or if it's erratic and unreliable with the ammo used
Russ Larges Posted - January 13 2019 : 07:41:03 AM
So there is no easy way to tell if my pistol is over gassed. I will just trust the manufacturer.
Thanks to all.
Ten Driver Posted - January 13 2019 : 02:25:48 AM
Russ, reference your original question, on a gun with an adjustable gas system, there are techniques for finding the right setting to use. A common one seems to be to find the gas setting where the bolt will begin to lock open on an empty magazine follower, then open up the piston a little more to guarantee reliability. Some folks will also use the pattern of the ejected brass to read where things are at.

Fixed gas systems are mostly influenced by ammo choice, as the guys already noted.
gw Posted - January 06 2019 : 12:17:08 PM
Stoner wasn't a big fan of a .22 rifle for the military

he got on board when it was adopted and had input on the necessary improvements, including improved ammo

dispite urban legend, the M14 was going to be the weapon, but estimates showed that production could not match the ramp up in Vietnam the Government had planned.

the AR15,with modern materials, could be made faster/cheaper

something McNamara could understand...

WR Moore Posted - January 06 2019 : 10:23:48 AM
Off topic, but several years ago I read that while Gene Stoner developed the original AR10, a couple of much younger guys actually did the AR15 development. Gene being deeply involved in other projects at that time. Since he developed the original concept, he got the lion's share of the fame/blame and royalties.

We really don't know what might have been, not all firearms designers are wonderful people. Way back when, my father came back from a trip to Springfield Armory (original) on a DOD project. I could tell he was deeply unhappy when he came back and when I inquired what was wrong, his reply was: "John Garand is an can't mention!" I took the hint and disappeared. Sorry I never went back at a later date to find out what the issue was.
gw Posted - January 04 2019 : 10:11:51 AM
chamber pressure dropping before the bolt unlocks is part of the issue with M4 extraction

this can be linked to ammunition as well

carbine length gas systems like the M4 operate at higher pressure, the gas port is closer to the chamber, this supplied gas is also hotter

higher pressure high temerature gas cause the gun to run hotter and dirtier

the closer to the muzzle gas can be tapped the simplier life becomes for DI rifles, the full length rifles run well

the carbine length gas systems have always been a tough nut to crack, Eugene Stoner was out of the loop when they came to be, maybe he could have got us here easier.

the mid length gas tube with the 16 inch barrel works a bit better

but the current M4A1 with heavy barrel runs and is a very good rifle

most of these bad things manifest themselves in full auto heavy use rifles, the current wars have been a good test lab to work out the kinks

most of this is how I recall it, been a long time since I had any real feedback on the M4 program

WR Moore Posted - January 04 2019 : 09:27:22 AM
I'd heard dwell explained in several weapons systems as the time between primer ignition (possibly maximum pressure-been a long time) and bolt unlocking. This applies regardless of system operation as it includes recoil operated systems also. If you don't have a gas system, you still have dwell time to allow chamber pressures to drop to an acceptable level.

You still confused Russ?
gw Posted - January 04 2019 : 02:30:41 AM
Dwell time is the delay from when the bullet passes the gas tube hole to the time the bullet exits the barrel. This stretch is actually pretty important, because the gas pressure used to cycle the action goes away almost immediately once the bullet has exited the barrel and broken the seal, so a bullet that exits too soon (too little distance between gas tube and muzzle) isn't giving the action enough time under pressure, commonly called "undergassing".

On the other hand, a bullet that takes its time about leaving (too much distance between gas tube and muzzle) is "overgassing". Overgassing sprays excess gas and debris into your action, making the gun run dirtier, but it doesn't damage the weapon itself as ARs vent excess cycling pressure through holes in the bolt carrier. A 16” barrel with a carbine gas system is almost always overgassed, as it has both higher gas port pressure and additional dwell compared to the original rifle-length system.

As a result of its shortened gas system, an M4 carbine operates at almost double the pressure of an M16 rifle, so Colt engineers had to not only master the dwell time, but find other ways to arrest bolt travel and reduce internal stress caused by the carbine's shorter gas system.

Colt was able to smooth things out and get the now-standard 14.5" barrel M4 running reliably by using a gas port at the 7.5" mark (to get the dwell time within limits), M4 feed ramps (to accommodate the increased cyclic rate), heavy-weight buffers (to compensate for the increased pressure), and all the other assorted nubbins and tweaks that are now commonly known to make carbine ARs more reliable.

WR Moore Posted - January 03 2019 : 7:53:35 PM
We're getting deep in the weeds here, but dwell time is more complex than just the distance from the gas port to the end of the barrel. You can also alter dwell by changing the weight of buffers in semi auto carbines. More weight=longer to unlock bolt. Full auto carbines have heavier buffers to reduce the bolt bounce/cyclic rate.

Gene Stoner described the AR gas system as a gas expansion system, rather than direct impingment. When the cartridge fires, you momentarily get maximum pressure, as the bullet travels down the barrel, the gas expands to fill the changing volume and the pressure drops.

Now, when the bullet passes the gas port, the volume changes again to include the gas tube & bolt carrier. The size of the gas port controls the rate/velocity at which the gas flows into the gas tube, which also helps control the pressure in the tube. Again, the pressure drops. When the bolt is fully unlocked, excess/remaining pressure is vented to atmosphere through the vents in the bolt carrier/gas block/gas tube.

However, in the carbine length gas system, the port pressure is a lot higher, the gas flow faster and the resulting pressure is higher in the bolt carrier than it is in the rifle.

I accidentally threw out my book on fluid mechanics so I can't get much beyond this as things can get real involved. I was real up on this stuff 40 odd years ago when I was designing muzzle brakes. Now if I look at the drawings, I've got no clue what the calculations mean. I'd just have to make it like the drawing.
gw Posted - January 02 2019 : 6:19:24 PM
dwell time comes into play as well

dwell time can be thought of as the time pressure is allowed to work on the bolt group until pressure decreases. this is determined by the distance from the port to muzzle

the M16 port size was optimized for that barrel length and dwell

when the carbine length gas tube was adopted, the barrel was short like the CAR15, the gun was undergassed and had reliability problems, port size was adjusted and gas tubes were lengthned (pig tailed) to add gas

later on when the M4 was developed the 14.5 inch barrel was used to to replicate the dwell time, distance from port to muzzle, of the rifle.

gas issues were solved for semi use, full auto fire produced other problems.

the 16inch civilian barrel with carbine gas system is over gassed, works in semi although some folks think it to much recoil for no purpose, it also breakes full auto bolts.

the 16 inch barrel with a rifle length gas tube is under gassed, shoots softer and is preferred by some. I've never seen one ran hard enough full auto to know better.

I have seen a bunch of M4s with 14.5 inch barrels ran to failure, they break bolts full auto.

I really haven't seen that with M16 rifle length barrels

if I wanted a full auo weapon I'd go that way

the Marines use piston driven HK 16 inch barrel guns with adjustable systems (M27)

however they operated those rifles, they are optimized for full auto and work well...
WR Moore Posted - January 02 2019 : 3:46:43 PM
Let me try to boil this down. You have two general approaches to gas systems: user adjustable and non adjustable.

The FN FAL/SLR is a good example of the first approach. It does give the user the ability to adjust for adverse operating conditions and ammunition differences. It also requires (OK, good idea. The thinking user will count the number of turn/clicks to fully closed if the firearm is functioning properly and return it to that setting after cleaning. A test is still a good idea.)that the system be calibrated after cleaning to ensure proper function. There is, obviously, plenty of opportunity to screw things up and end up with a prematurely worn or non-functioning rifle.

Added edit: there are adjustable gas system with limited settings. Usually normal, adverse conditions (more gas) and off (no gas).

The US has primarily stayed with the non-adjustable gas system. This keeps the user from messing up what should be a properly functioning firearm. For this to work, the system does trap and use more gas than absolutely necessary to operate a clean firearm with good ammo under good conditions. This results in a bit more enthusiasm in operation under those conditions, but doesn't require the user to make adjustments when the firearm is dirty, ammo may not be top of the line and/or it's been exposed to mud, blood, rain & other adverse conditions. The hair splitters refer to this as being "over gassed".

Said hair splitters (some of whom insist that there's 50,000 psi in that gas tube) want the absolute minimum gas in the system to operate the bolt to minimize recoil(the .223 has recoil?) for various reasons. These reasons can range from theoretical advantages in gun games to being able to load really stout loads for long range shooting without beating up the rifle. I've been using the AR system since about 1970 or so, when issued, as issued. I have little patience for the kind of nit picking some people do. The AR system is somewhat self regulating as to pressure, but like all systems is designed around a range of acceptable port pressures.

Now then, the various systems. The original design was the full size firearm with 20 inch barrel and rifle length (~12 inch) gas system. The carbine had development issues, in part because the reduced length of the gas system roughly doubled the gas pressure in the bolt carrier. Yes, the two systems have different diameters of gas ports. The short system still operates at higher pressures and is more violent in operation.

The mid-length gas system (~9 inch) is what I went with when I spent my own money. Less pressure in the system, less violence of operation, more forgiving of ammunition variations than the carbine length system.

I've got no knowledge of the pistols, I expect they're kinda finicky. There were several US Marshalls at the last school I went to with 11 inch ARs they generally ran suppressed. They functioned flawlessly, but when I talked to them they indicated they had a guru that worked over the guns after purchase to ensure proper operation before they were issued.

BTW, from what I've observed, you don't want to bet your butt on gas blocks that aren't pinned on. I've seen a couple secured by set screws/bolts work loose. The results are spectacular when the gas block blows forward. Forums © 2002-16, Inc. Go To Top Of Page
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