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MistWolf
05-08-10, 22:07
From what I've read here and a couple of other places, the shorter the gas tube, the more violent the operation of the action. Especially with the combination of the carbine length gas tube and the 16" barrel to increase "dwell time" caused by the greater distance from the gas port to the muzzle over the 14.5" barrels. From what I understand this increase in dwell time increases the pressure.

I also understand that the shorter the barrel, the shorter the gas tube, the larger diameter the gas port needs to be drilled to facilitate full functioning.

This seems contradictory. If the port needs to be enlarged that means more gas is being let into the gas tube. Not only does a larger hole let more pressure into the tube but more volume as well.

The Garand has it's gas port near the muzzle and nearly zero dwell time. Yet, the M14 had it's gas port moved closer to the breach to reduce the violence of gas pressures that sometimes bent Garand op rods. The M14 has increased dwell time compared to the Garand, yet it's gas system was shortened to reduce the violence of action.

I have no doubt the rifle with it's longer gas tube is smoother shooting than the carbine. (I haven't shot many carbine length ARs to compare to my rifle.) The recoil impulse of the firing of the bullet is closer in time to the operation of the action with the shorter carbine than it is with the longer rifle.

Does the increased dwell time really increase pressure of the gasses diverted to the action of an AR? Or is the sharper perceived recoil because it allows a sharper pressure spike? Or something else?
P2000
05-08-10, 22:23
I'm no expert, but I think it is because 10.5 or 11.5'' barrels also use CAR length gas tubes, making them have less dwell time than a 14.5''.
500grains
05-08-10, 22:38
I cannot answer your question, but I would like to mention that it is my understanding that a mid-length gas system works more reliably with a suppressor. Not sure why, but it would seem to be related to the issue you raise.
MistWolf
05-10-10, 17:37
No one has any answers? Theories? Questions? Comments? Snide remarks?
sinister
05-10-10, 17:49
http://www.armalite.com/images/Tech%20Notes/Tech%20Note%2048,%20Barrel%20Design,%20Heat,%20and%20Reliability,%20030824%E2%80%A6.pdf

The M14's gas system is different from the M1's. The M14 uses a gas piston while the M1 has direct impingement on the op rod.
markm
05-10-10, 18:53
The gas tube length has little/nothing to do with it. It's the gas port's distance from the chamber. Gas tube's length is misleading.... as though the pig tail style gas tube would make significant differences just because it's longer... you still have the same high port pressure.

Port pressure drops off fairly dramatically as you push the port farther down the barrel. And as pointed out... you generally need a larger port if you have less barrel length after the gas port.

For example. A rifle gas system on a 20 inch barrel works fine. But you'd likely have to open the ports some if you cut that same barrel down to 16" with the exact same gas system. Dig?
Col_Crocs
05-10-10, 19:14
The gas tube length has little/nothing to do with it. It's the gas port's distance from the chamber. Gas tube's length is misleading.... as though the pig tail style gas tube would make significant differences just because it's longer... you still have the same high port pressure.


Well put.
ChicagoTex
05-10-10, 22:42
dwell time doesn't increase pressure, it increases duration.

In the gas system of an AR you have three factors:

1. Pressure (the pressure of the expanding gasses at any given point in the barrel)
2. Volume (dictated by gas port size)
and 3. Duration (How long is gas being siphoned through the gas tube, dictated by dwell time)
bkb0000
05-10-10, 22:50
its a variable formula and each element exists on a sliding scale.. bring one one, push the other out, and not necessarily proportionally.