Shortest barrel length upper?

[fledge]

If you want to experiment, SMOS makes a 16” match barrel with rifle gas.

https://www.smosarms.com/products/sm...nt=44538212999

[Clint]

My admittedly limited understanding is the longer the gas system the lighter the recoil. So I’m curious about max length possible with various barrel lengths.

A longer gas system is better until it's too long.

A good way to look at it is this:

For each gas system length, there is a range of barrel lengths that will work and an even smaller range of barrel lengths that are Optimal.

[WeepingAngel]

OP, I’m going to assume for a second that you are still new, or newer to the AR system. If I am incorrect, I apologize. The issue with running the rifle length gas system in a 16” barrel is the dwell time. That is the time from when the bullet passes the gas port, to when it exits the barrel. Think about this, the gun doesn’t start doing any work until the gas system is “charged”. Ie, when the bullet passes the port, and gas starts back to unlock the bolt. The entire process has to happen before the bullet leaves the barrel, and your pressure falls to zero. Once the bullet leaves, you have no pressure left in the system. I would think, and I’m not 100% here, but I believe you would have to run a bigger gas port doing it this way, due to needing a bigger/faster pressure charge to offset your shorter dwell time. By running a larger gas port to offset the shorter dwell, you’ve likely given up the soft, smooth recoil impulse inherent to the rifle length system. Therefore, the middy would, and btw does make more sense. I have personally seen guns set up like you are asking. Rifle length gas systems on 16” guns. The 2 I have seen, functioned fine. I don’t know what size gas port the barrels had. In this day and age, many companies run gas ports that are too big. This is so guys who insist on running crap ammo will have functioning guns when they shoot underpowered PMC or the like. If these guns had gas ports that started off too big, that would explain why the 16” rifle length worked. On a properly gassed barrel, I think you’d be on the ragged edge of reliability. You’d likely have to run a lighter buffer, and full power ammo.

In the end, it’s your gun, you can do as you please. Just remember that the system is set up to run inside certain parameters. Step outside those, and reliability may suffer. For ME....16”= middy, 18”= Rifle length.

Good stuff, Vegas. I've read a few explanations as to the dwell time issue; that's one of the clearest I've seen. Thanks for posting that.

[Rayrevolver]

I have a suppressed only 11.5 mid length. It works with an adjustable gas block to close down the ginormous gas port.

It's not a go to war setup like other vetted configurations.

[Diamondback]

A longer gas system is better until it's too long.

A good way to look at it is this:

For each gas system length, there is a range of barrel lengths that will work and an even smaller range of barrel lengths that are Optimal.

Clint, if you have time sometime would you consider working up a chart with the "Green" (optimal) and "Yellow" (workable) zones for the four common gas lengths? Some of us are into the science and tech of how this stuff works and why things get done the way there are, rather than just "milspec for the sake of milspec Because The TDP Said So"--and some are more in the Patrick Sweeney camp of "milspec = Minimum Acceptable rather than end-all be-all."

[MisterHelix]

The KAC SR15 uses an intermediate (>mid, <rifle) gas tube on a 16" barrel. Mine is a soft shooter.

Then again, it's a carefully engineered factory combination, vetted and tested with NATO spec ammo. They're available. If that's what you're after, get one.

If I were assembling my own, I'd just get another Noveske 16" middy (or whatever) and call it a day.

[26 Inf]

Does anyone know of an upper with a rifle length gas system with a barrel shorter than 18 inches? Is that even possible?

And what is the shortest possible barrel with a mid-length gas system?

Yes it is. Just get someone to cut down a barrel with a rifle length system to 15.5 (15.494) from the breech if you want to put a muzzle device on it.

https://rifleshooter.com/2014/05/bui...-from-a-blank/

That is a whole 1/2" shorter than a 16" dissapator and gives you the added expense of either pinning or welding the muzzle device or registering it as an SBR, unless you want a pistol.

That will give you about 2.3 inches of dwell after the port. The 20" M16 has 6.8 inches of dwell after the port on it's rifle length system; the 14.5" M4 has 6.7 inches of dwell after the port on it's carbine length system. Their is a reason for that:

As a general rule, you get best function with at least 5" of barrel in front of the gas port. Gas port size is also used to adjust function when a non-typical gas system length is used. As a general rule, barrels with smaller diameter ports and more barrel out in front work with a wider range of ammo than barrels with a larger port that's closer to the muzzle.

http://www.ar15barrels.com/prod/gas-systems.shtml

[Clint]

Clint, if you have time sometime would you consider working up a chart with the "Green" (optimal) and "Yellow" (workable) zones for the four common gas lengths?

We do have a chart and use it most days.

It has the following gas system lengths charted against a range of barrel lengths for each system, highlighting the Optimum configurations.

Pistol
BRT PDW
Carbine - 11.5"
Mid
BRT EXT - 16"
Intermediate
WOA
Rifle - 20"
Rifle+2

It contains proprietary technical data, including timing ratios, gas drive and gas port sizes.

As such, it represens a significant investment, and wont be posted on the open internet in its current form anytime soon.

I may be able to drop a few breadcrumbs along the way...Rest assured any barrel with BRT stamped on it will be an OPTIMUM configuration.

[triggerjerk]

Digging the dwell expl bigtime, but in another thread ime thinkin someone mentioned rearward inertia of buffer after bullet exit pressure drop. Anything to it? A simple mind needs yet another clue....

[archad]

I had an instructor who made a 16 inch gun out of a Colt rifle barrel that no one wanted in the Sheriff's office Armory. It ran fine.

For smoothness, however. Nothing beats the BCM 14.5" middy with an H buffer.

A BCM 14.5" runs smooth for sure I have one. A KAC/intermediate/optimal length is a little smoother IMO.

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