Questions on how the gas system works on AR's

[brentn]

Continuing to read up on the M15 rifles and I have a couple of questions.

Where exactly does the gas port start on the barrel? is it up by the sight?

If you had a 20" M15A2 and bought an 11.5" barrel for it, would you have to adjust the system so that it cycles properly? would it require a weaker recoil spring becuase there is not as much barrel length for gas pressure?

Would appreciate any input, just trying to understand how this exactly works. My guess so far is that the gas from the cartridge builds up behind the bullet before it leaves the barrel. Somewhere down the barrel, similar to a compensated barrel say on a glock, is a hole. Instead of pushing gas into the atmosphere its directed via the gas tube which in turn goes into the upper reciever. The gas tube attaches to the "gas key?" on the bolt, which in turn, the pressure pushes the bolt back with so much force that a round is instantly cycled.

Do I got this right?

 

[brentn]

Another question as well, is it necessary to clean the gas tube? I think I remember reading the armalite manual (yea even though I don't own one, I have downloaded it and read it thoroughly just to satisfy some interest) that it isn't necessary or something.

thanks

 

[gotm4]

Read this http://www.m4carbine.net/showthread.php?t=94

 

[sacp81170a]

Another question as well, is it necessary to clean the gas tube?

In 27 years of shooting M-16's/AR's I have yet to clean a gas tube with the exception of running a pipe cleaner into it from the receiver to make sure that end was clear. No mention of it in any of the training I've had.

 

[brentn]

That link is pretty technical, and covers quite a bit about extraction and detailed bolt function description.

I answered one question though, the gas system is regulated purely only by the location of the gas port on the barrel. Point being, if you were to buy an 11.5" barrel it would be a drop in with no mods really necessary.

One thing I don't understand is why there is a greater pressure on a shorter chamber-gas tube length? Is it because the distance from the bullet, to the end of the barrel is greater on a carbine gas port location, thus building more pressure?

Eh, sorry if I'm not fully getting this, just trying to figure things out. Appreciate your patience.

 

[Shear_stress]

One thing I don't understand is why there is a greater pressure on a shorter chamber-gas tube length? Is it because the distance from the bullet, to the end of the barrel is greater on a carbine gas port location, thus building more pressure?

Actually, the opposite. The carbine gas port is closer to the chamber, and pressure is higher than on the rifle length gas system. Fluid flow moves from high pressure to low pressure.

 

[Bartholomew Roberts]

There are also some excellent discussions of the AR gas system in the Readining Library - including a 1968 training film showing its operation.

 

[DMK]

The carbine gas port is closer to the chamber, and pressure is higher than on the rifle length gas system. Fluid flow moves from high pressure to low pressure.

Exactly right. Not only is there a pressure issue with the shorter gas system, but there is a timing issue too. Since the gas port is closer to the chamber, the bullet passes the port sooner. This allows less time for chamber pressure to reduce and for the cartridge case to relax (the cartridge expands against the chamber wall from the heat and pressure of the gasses expanding within it). This makes it more difficult to pull the cartridge from the chamber which puts more load on the extractor and could slow the extraction of the cartridge, or even allow the extractor to 'jump' the rim of the cartridge, leading to a jam(failure to extract).

 

[gotm4]

One thing I don't understand is why there is a greater pressure on a shorter chamber-gas tube length? Is it because the distance from the bullet, to the end of the barrel is greater on a carbine gas port location, thus building more pressure?

Eh, sorry if I'm not fully getting this, just trying to figure things out. Appreciate your patience.

There's greater gas pressure in shorter gas systems for two reasons. One is that a gas tube on a carbine length system is 5" shorter (thus closer) than on a rifle length system. Two on some barrels say a 11.5", 12.5" and 14.5" the gas port is the barrel is larger than on a 20" rifle barrel which makes for more volume at a sooner rate (and a faster cyclic rate). Getting shorter barreled ARs to run right vs. a 20" AR is a fine science. Because everything has to happen more quickly for everything else to work at the right time it can be trouble. Shorter than 20" ARs have more things that wear out/break more often. Bolt life for instance is shorter on M4s for this reason.

 

[DMK]

The distance from the gas port to the end of the barrel is important too. That controls the dwell time of the gas against the bolt carrier. If the port is too close to the muzzle, pressure will drop before the bolt carrier has moved far enough (once the bullet has left the barrel, gas pressure will vent though the bore, dropping the operating pressure to zero).

The crutch for this is to open the gas port more, putting more pressure on the bolt carrier, making it move faster. Obviously, this isn't the best solution to the problem, causing other problems of it's own.

 

[Navy87Guy]

Try this link: http://www.barnesengineering.com/AR15animation/index.htm

To me, a picture is worth a thousand words!

Jim

 

[wdlsguy]

How it Works- The AR gas operation and how everything works in harmony.

 

[brentn]

awesome material guys, cleared up a few things.

One thing though, is it possible to have a shorter 11.5" barrel work flawlessly? it doesn't seem so after reading all that. Sounds like there would be so much pressure that you would get ripped case rims, broken extracters and pummelling of the bolt into the back of the recoil spring tube.

Is it common for guys with too much gas pressure to put regulators on the gas tube? is this a good idea?

I appreciate the replys!

 

[wdlsguy]

is it possible to have a shorter 11.5" barrel work flawlessly?

I believe so. My 11.5" Del-Ton upper hasn't missed a beat yet.

 

[Jim K]

The pressure in any firearm is greatest closest to the chamber.

Think of a roller-coaster with a steep up ramp, a peak, and then a down ramp gradually approaching the ground. Superimpose that on a picture of the barrel and you will have a rough idea of the pressure curve as it goes from zero at ignition time, to a peak just as the bullet starts to move, then a decrease as the powder burns and the bullet moves up the barrel.

So the closer to the chamber the gas is taken off, the higher the pressure will be. This can be compensated for by making the gas port smaller so less gas is ported out, but that must be done precisely. Too small a port can result in failure to extract; too large a one can mean rims and heads torn from cases and gas let loose in the action.

Jim

 

[brentn]

wdlsguy, what have you done to your 11.5" that others have not done?

Cause all i hear it seems are complaints about shorter barrel lengths with FTE's. I can understand that everyone with a shorter barrel length has more wear on the bolt etc but if they're having FTE's how come yours does not?
Just trying to see if you've prepped yours in a way or something, thank you!

 

[Chipperman]

My M16 functions fine with both a 14.5" and a 10.5" upper. When I first got it, I was having all kinds of problems. Some of them related to old springs, others to the gas system.

The shorter uppers will often have problems with "Bolt Bounce" (See DMK's first response). I put in a heavy buffer to cure this problem.

Now it operates reliably with both 14.5" and 10.5" uppers.

 

[brentn]

Thats the info I need to know thanks buddy!

 

[dagunner]

I would like to know what the fascination with AR barrels that are shorter than 16" anyway.Any barrel shorter than 16" has to have acuracy issues.

 

[DMK]

Any barrel shorter than 16" has to have acuracy issues.

Why would it have accuracy issues?

Short barrels are not less accurate. That's a common misconception. They only have less velocity (and a shorter sight radius, depending on the rifle).

 

[wdlsguy]

what have you done to your 11.5" that others have not done?

I sent my 16" upper to Adco Firearms to have it cut, threaded and crowned. That's it.

I haven't tried running it full-auto, since it has an AR-15 bolt carrier, and I don't have a full-auto lower.