So about the 6920 and 16” carbine gas...

[17K]

My first AR in ‘01-‘02 was a Colt, presumably a 6920, but I’m not sure if it was or not.

I shot many rounds through that gun with absolutely zero issues until I got my my first personally owned AR, an ‘04 LE6920HB, that I put over 15K rounds through before I moved on to an ‘09 6920 because I wanted the lighter barrel and .154” fcg pins.

Had, and still have several 6920s along the way and the two ARs I depend on day in and out are built on 6920 OEM-2 uppers.

I shoot all manners of brass cased ammo, all year long in temps ranging from the ‘20s to a little over 100.

I don’t full auto or suppressed and I actually lube the things even if I don’t clean them. Ever.

I can honestly say that in my two decades of 16” carbine gassing shooting I have never had a failure, or broken a part that wasn’t a magazine.

I have had a couple of BCM 16” mids over the years as well and even with a shot timer I never could quantify any difference in performance, and actually feel like the Colts were softer shooting, all things being equal.

I believe that the mid-length port position is a better mousetrap, but is it really anything more than an almost hypothetical advantage?

I have been thinking about it for a couple of days because I have 3 Daniel Defense 16” mid barrels sitting here that I want to put together, but I also have a few more OEM-2 uppers that are ready to go with just a handguard.

Has anyone really had a failure with a 6920 and said ‘dammit! if that gas port was two inches farther down there, that never would’ve happened!’

I limit this question to Colt 6920 barrels only because once you leave that brand, things only go one direction and it’s not the right way.

[TomMcC]

The difference is quite subtle and subjective. Felt recoil can feel ever so slightly lighter. Many other factors come into play also. Like the weight of the rifle. The weight of the BCG. Does it have an effective brake or are you running a FH. My carbine gas gun does feel a bit sharper in it's action than my mid and rifle length. But my rifle length is set up for 3gun, so I'm running an AR-15 bolt carrier, and a pretty good brake. My carbine is actually heavier than my rifle length. So many variables involved that change the feel of a gun.

[okie]

It makes a slight difference in terms of gas port erosion and that's all. The placement of the gas port on carbines is basically arbitrary, because it had to do with the already existing M16 bayonet. That is, the M16 bayonet was designed around the M16, whereas the carbines were designed around the M16 bayonet. So basically bass ackwards, especially considering that bayonets don't make much sense on carbines in the first place. People assume there were like tests or at least some engineering calculations or something that went into determining the barrel length and port placement. But nope, it was completely arbitrary.

And mid is equally as arbitrary. Well somebody along the way decided they wanted a bayonet on a 16 inch, and that's how we got the mid. Then they started justifying it by making up all kinds of nonsense about how it's softer shooting blah blah blah.

Okay, in an over simplistic sense, it does help to put the port out as far as possible. But it's a give and take situation. Put it too far out and it's going to choke. Put it too far back and it's going to rip case heads off or just not work at all. The longer the barrel, the more room you have to play with and get the best of both worlds. The shorter the barrel the less dwell time you can afford, meaning you have to enlarge the port.

And that's the rub right there. Slight movement of the gas port can be offset by slight changes in diameter to the port, and that will achieve basically the same results. So if your carbine is shooting too fast and hard, you can simply reduce the port size.

Or just move up to a heavier buffer.

Where carbine length makes sense is that you can slow the bolt down with a heavier buffer, but you're still getting the thrust of that higher pressure gas. So if the chamber is a bit dirty or whatever, that's not going to matter as much, which is why people find carbine to be more reliable.

In terms of felt recoil, barrel length is the only thing that's going to determine that. The shorter the barrel, the less time you have to make everything happen. So the action will be moving faster, and feel snappier.

I don't know what the perfect length would be, but both carbine and mid are in the ballpark. I don't personally care because anything longer than 10.3" needs to get chopped anyways. But on 16" barrels, I've not had issues with either length, and there are valid reasons why each would be "better."

[Pappabear]

I have had zero issues with my Colts. I currently have a 6945 10.3 that runs and eats anything. I also have the skinny barrel Colt 16’ I think it’s the 6700, and I’ve never said “ dam I wish that port were further out”. Both run great, and my BCM’s, LMT’s are the same.

I buy into the mid length story, but feel it’s nice not necessary. Like A5 buffer vs H2 n most cases.

IMHO buy quality gear and it doesn’t much matter where the port lands, put the Pony on it and its even better.

PB

[MistWolf]

...And mid is equally as arbitrary. Well somebody along the way decided they wanted a bayonet on a 16 inch, and that's how we got the mid. Then they started justifying it by making up all kinds of nonsense about how it's softer shooting blah blah blah.

Some years back, there was a thread discussing this very subject. One poster who was there during development of the middy explained Armalite did so as an improvement over the 16 inch carbine gas. He said the fact it also allowed the use of a bayonet was happy coincidence for the ad guys.

if your carbine is shooting too fast and hard, you can simply reduce the port size.

Or just move up to a heavier buffer.

It doesn't work that way. For each barrel configuration (length, port location, suppressed or unsuppressed) and ammo type, there is one port diameter that works best. If the port size is wrong, changing buffer weights won't fix the problem.

The reverse is also true. If the buffer weight is wrong, changing the port diameter won't fix it.

Gas drive and reciprocating mass have to be correct.

Where carbine length makes sense is that you can slow the bolt down with a heavier buffer, but you're still getting the thrust of that higher pressure gas. So if the chamber is a bit dirty or whatever, that's not going to matter as much, which is why people find carbine to be more reliable.

When the military tested the carbine gas system against the middy, they found the middy to have an edge in reliability. (I don't know if the carbine gas ARs had 14.5 or 16 inch barrels. The middys were 16 inches.)

17K, if you have uppers with Colt barrels, run'em! If a Colt 16 inch carbine gas system barrel is 95% perfect, it would be 97% perfect if it were a middy.

[17K]

I do. I generally shoot about 5K of 5.56 a year and it’s all out of a 6920 barrel.

I actually have a BCM 16” carbine gassed upper and it’s been 100% through a couple cases of Q3131 over the winter. It is a little harder on brass and seems to shake the dot more than my (identical) Colt.

[okie]

Some years back, there was a thread discussing this very subject. One poster who was there during development of the middy explained Armalite did so as an improvement over the 16 inch carbine gas. He said the fact it also allowed the use of a bayonet was happy coincidence for the ad guys.



It doesn't work that way. For each barrel configuration (length, port location, suppressed or unsuppressed) and ammo type, there is one port diameter that works best. If the port size is wrong, changing buffer weights won't fix the problem.

The reverse is also true. If the buffer weight is wrong, changing the port diameter won't fix it.

Gas drive and reciprocating mass have to be correct.



When the military tested the carbine gas system against the middy, they found the middy to have an edge in reliability. (I don't know if the carbine gas ARs had 14.5 or 16 inch barrels. The middys were 16 inches.)

17K, if you have uppers with Colt barrels, run'em! If a Colt 16 inch carbine gas system barrel is 95% perfect, it would be 97% perfect if it were a middy.

I’m skeptical about Armalite’s version of events, but I will leave it at that.

“Correct” is pretty subjective. Different flavors of ammo and muzzle accessories make that something that the end user just has to figure out for themselves. Which is why I try to copy Crane as much as legally possible. Might not be perfect but at least it’s the devil we know.

As far as the testing on mid, I just adopted the assumption that it was an M4 vs a commercial mid 16, in which case the additional barrel length could explain the slight increase in reliability. I don’t put too much thought into it though because the difference isn’t enough to rule out other variables. Like I said though the variety of ammo and cans out there makes “correct” too subjective. “Reliability” is also a subjective term. A change in length could be dependent on the configuration in question. One configuration might prove more reliable with a certain length, while another might prefer another length. I think it’s six of one, half dozen of another.

It does make sense to opt for carbine though because it leaves your options open to have it cut down by adco. I always recommend people go with an le6920 so when they inevitably want to go SBR they can just have adco chop the barrel for the low low price of 75 dollars.


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[AndyLate]

It makes a slight difference in terms of gas port erosion and that's all. The placement of the gas port on carbines is basically arbitrary, because it had to do with the already existing M16 bayonet. That is, the M16 bayonet was designed around the M16, whereas the carbines were designed around the M16 bayonet. So basically bass ackwards, especially considering that bayonets don't make much sense on carbines in the first place. People assume there were like tests or at least some engineering calculations or something that went into determining the barrel length and port placement. But nope, it was completely arbitrary.

Completely wrong and easily disproven. Carbine gas system came from the CAR-15/XM177 which had 10.5 - 11.5" barrels.

The M4 has a 14.5" barrel and that is the length required for a bayonet and carbine gas. I would assume thats why the length was chosen, but it was well after the carbine gas system development.

On a personal level, I prefer a midlength gas system on a FSB gun because carbine handguards are cramped as heck.

Andy

[okie]

Completely wrong and easily disproven. Carbine gas system came from the CAR-15/XM177 which had 10.5 - 11.5" barrels.

The M4 has a 14.5" barrel and that is the length required for a bayonet and carbine gas. I would assume thats why the length was chosen, but it was well after the carbine gas system development.

On a personal level, I prefer a midlength gas system on a FSB gun because carbine handguards are cramped as heck.

Andy

What you have to take into account though is that the first carbines were chopped M16s, with approximately 15 inch barrels. The development of the carbine gas tube was in response to the failure of those to cycle reliably.

[AndyLate]

What you have to take into account though is that the first carbines were chopped M16s, with approximately 15 inch barrels. The development of the carbine gas tube was in response to the failure of those to cycle reliably.

No. You stated the carbine gas system was designed to allow a bayonet to be used with a short barrel, which is simply incorrect.

Andy