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

[Clint]

Luckily, NSWC-Crane did a little test on this a couple years back.

Our discussion and link here.

NSWC-testing-on-gas-systems-CARBINE-VS-MID-LENGTH

[17K]

Pretty interesting report. I remember reading parts of that a few years ago and the failure rates really started to show up in either the arctic or the heat soaked part of the test, but I never could deduce what exactly they were doing to make those guns choke so much.

[1168]

What is most interesting to me in that report is that the bolt speed delta when adding a suppressor is higher in the mid gas than carbine. Which seems to play out in my own limited experimenting so far. But, its the opposite of conventional wisdom.

[Uni-Vibe]

When I take my old A2 Bushmaster to the range, people want to shoot it. They fire a few rounds, and smile, and say "smooth."

The cycle is a bit smoother with 20" gas length than on my 6920. But both work if they are set up correctly.

[okie]

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

Well as far as I know, that's the only explanation that's ever been offered. If you have documentation to the contrary, though, don't hesitate to post it. I've not made any secret that I have a serious hard on for Colt, so anything scientific that would suggest the carbine system was actually well thought out by engineers would be welcomed by me.

[Clint]

The carbine gas length was designed around 1965 for the XM177, which had a 10" barrel.

The barrel was later lengthened to 11.5", which restored proper gas system proportions and increased reliability.

The M4 carbine came about much later.

In the early 80s, the .GOV wanted a carbine version of the M16.

The XM177 was offered up and tested.

A bunch of changes were made including furniture, adding the cartridge deflector and lengthening the barrel to 14.5" to allow fitting of the bayonet, which also improved ballistics.

The gas system was not lengthened, as it would have precluded use of the bayonet, but it should have been changed to what is now MID length in order to maintain proper proportions and peak reliability.

The end result was the M4 carbine, which was adopted in 1991.

Well as far as I know, that's the only explanation that's ever been offered. If you have documentation to the contrary, though, don't hesitate to post it. I've not made any secret that I have a serious hard on for Colt, so anything scientific that would suggest the carbine system was actually well thought out by engineers would be welcomed by me.

[vicious_cb]

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.

Congrats, you have learned through first hand experience what almost 2 decades of internet speculation couldn't figure out. That there is no significant real world difference between carbine vs midlength gas systems in 14.5-16" barrels. Ive shot out 3 carbine barrels over the years: LMT, Colt 6920 and S&W and slowly replaced them with midlengths and havent noticed a real difference in reliability or speed.

Is it a better mouse trap? Yes, physics doesnt die.
Should you go out and replace your carbine lengths with midlengths? No, its a waste of money.

[okie]

The carbine gas length was designed around 1965 for the XM177, which had a 10" barrel.

The barrel was later lengthened to 11.5", which restored proper gas system proportions and increased reliability.

The M4 carbine came about much later.

In the early 80s, the .GOV wanted a carbine version of the M16.

The XM177 was offered up and tested.

A bunch of changes were made including furniture, adding the cartridge deflector and lengthening the barrel to 14.5" to allow fitting of the bayonet, which also improved ballistics.

The gas system was not lengthened, as it would have precluded use of the bayonet, but it should have been changed to what is now MID length in order to maintain proper proportions and peak reliability.

The end result was the M4 carbine, which was adopted in 1991.

I think it's much more likely that the carbine gas tube was designed with an approximately 15 inch barrel in mind, and that the M4 was more of a coming full circle situation.

However, if you want to take the opposite view, it's still arbitrary, which makes mid just as arbitrary. It's just a difference of believing it was placed to accommodate an FSB on a 10 inch barrel, vs. believing it was to accommodate a bayonet on a longer barrel. Both views could even be correct. They may have settled on an approximately 8 inch gas tube to accommodate the bayonet on a 605 style carbine, then realized they could shorten it slightly and be able to accommodate a 10" barrel using the existing FSB.

My point is though that either way there was never at any point this sit down moment where they were weighing the pros and cons of port placement. At every turn, it was placed where it was convenient to do so, then they fiddled with port size and buffer weight to dial it in. Even on the original M16, its placement was somewhat arbitrary, because the size of the human hand dictated that the resulting bayonet was going to be a certain length.

[JediGuy]

Clint posted before I could. His explanation is accurate. The carbine gas system was designed for a 10” barrel, which didn’t work very well, and was adapted to an 11.5” barrel, which did and does. Colt simply didn’t change the gas port location when revising their carbine offering to have a 14.5” barrel.
Colt later added another 1.5” to offer this new carbine to the civilian market. It is that simple.

[AndyLate]

I think it's much more likely that the carbine gas tube was designed with an approximately 15 inch barrel in mind, and that the M4 was more of a coming full circle situation.

However, if you want to take the opposite view, it's still arbitrary, which makes mid just as arbitrary.

Jeez, dude. You want us to ignore the sequence of real events and believe your idea of what happened? Why don't YOU provide documented proof of your claims?

Andy