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Thread: Extreme Cold Gas System Operation

  1. #21
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    Quote Originally Posted by Clint View Post
    The fact that even suspect configurations warm up after a few rounds points to either chambers warming up, restoring ammo power or cold gas system components warming up and no longer "robbing" energy from the gas.
    As foxtrotx1 said chamber pressure is more about the temperature of the powder than temperature of the barrel. Of course substandard lubrication will allow better functioning of the action when warmed up, but the chamber pressures and thus muzzle velocity will be significantly less if the powder has no time to warm up in the barrel before being fired.

  2. #22
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    With less barrel after the gas port there is less time for the bullet to act like a plug and therefore less time that gas is being forced through the port. That has always been my reasoning anyway.

    Quote Originally Posted by Clint View Post
    This all circles back around to the concept of the low dwell time configurations being less robust / more sensitive to variations in ammo or environmental conditions.

    As to WHY short dwell configurations are less robust:

    I'm not 100 sure at this point.
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  3. #23
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    But how much time? The extra two inches is a fraction of a micro second and the carrier doesn't even begin moving until the bullet has exited the muzzle. I suspect the gases don't even reach the carrier until the bullet has left the house. What that extra length changes is how long it takes the barrel to blow down to atmospheric pressure at the point of the gas port. The pressure in the bore doesn't drop to zero the instant the bullet uncorks it. Until the expanding gasses drop to atmospheric pressure, the pressure is pushing gas out the muzzle and through the gas port and that takes time.

    Let's say that the gasses do reach the expansion chamber before the bullet exits. Let's say the gasses hit the expansion chamber when the bullet is 2 inches past the gas port. The extra barrel length would not matter because the bullet will always be 2 inches from the gas port when the gasses hit the expansion chamber regardless if there were 6 inches or 60 inches of barrel after the gas port.

    But what if the gasses don't instantly fill the expansion chamber? What if they are fast enough to reach the expansion chamber but slowly fill it so the carrier doesn't reach full pressure until after the bullet exits? Then the extra time the bullet is in the barrel will make a difference, right? Probably not. Remember the bullet is traveling at over 2500 fps and is accelerating as it passes the gas port. The time it takes a bullet to travel 2 inches at those velocities is less than a micro second, not enough to have any real affect on the pressure in the action.

    Another thing to consider is that the 300 BLK must use a carbine length gas system or shorter to function properly, even if the barrel is lengthened to increase "dwell time". Take a look at how short the gas system is in the M1 carbine. It's because if it were any further down the barrel, the gasses would not have enough pressure to operate the action- and that is with the piston right there in the gas block!

    It's not how long the bullet keeps the bore sealed that keeps the pressure up, it's how long the pressure stays at operating levels at the gas port after the bullet exits that matters
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  4. #24
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    Quote Originally Posted by K.L. Davis View Post
    Okay... this is how it happened.

    In 1986, I started playing with different barrel/gas system lengths... finally, about 10 years or so ago, I took several barrels and started drilling and welding gas ports... cutting barrels back... opening gas ports... and measuring pressure at the gas port in every configuration I could - I ended up with pressures and lock times for 22, 20, 18, 17, 16, 15.7, 15.5, 15, 14.5, 13, 12.5, 12, 11.5, 11.5, 10.5, 10.3, 10, 9, 8, 7.5 and 7 inch barrels with gas ports on each barrel starting at ~2" from the muzzle and every inch back to ~5" from the chamber.

    During this testing, the freeze test was done on a lot of the guns that seemed to work good... of course a great many of the test configurations were not even worth trying to get to run.



    The 16" Middy is the ideal carbine (my opinion), when properly built from good parts, it runs like a clock.

    Systems that fail are pretty predictable... I wrote this in another thread - but it is to the point:

    The only real important pressure is the max/operating pressure inside of the BCG - that pressure is a function of the following:

    Gas Pressure at the port
    Port Size
    Barrel diameter
    Time the pressure stays high (dwell)
    Lock Time - the time the BCG remains locked, while pressure is allowed to build

    But yes, dwell time is one of the most contributing factors.


    Eric D nailed it pretty well... if you have a proven configuration that runs well at 70* - I would not worry about temps dropping to the teens... the only time I would even factor that in would be making a cold rifle/ammo shot at long range...

    The bottom line is this: systems that work are the ones people stick with, but often you get some weird or even retarded designs from the "can't make it better, make it different" crowd - if you are looking at a barrel and gas system combination that you have never heard of... it might be worth asking around to see what others think. M4C has more than a few folks that are able to tell chicken shit from chicken salad.
    Excellent post and thank you for the information. How does the 14.5" middy perform in your testing?
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  5. #25
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    Quote Originally Posted by BufordTJustice View Post
    Excellent post and thank you for the information. How does the 14.5" middy perform in your testing?
    Thanks... I was partial to a ~15.0 / 15.5 barrel for the middy gas system, the reason I say the 16" Middy is the best choice, is that it is not an NFA choice...

    The 14.5 middy does okay - lots of folks are running them, my personal opinion is that it is leaning on the short side of a barrel for a middy... but not so much I would ever discount it. I made up a barrel I called the "Aardvark" - it is an M4 with a mid-length gas system... the gas tube ran through the front sight to a gas block located in front of the gas block. I guess I like that setup, I still have one...



    But I think the barrel is 15" on it.

    The best 14.5 gun I have ever shot was one we did up with some guys in Ohio - it was for all intents and purposes a regular old M4, but the gas port was in the front "leg" of the FSB... about 1.5" forward of the carbine location... so about 0.5" short of the middy. It ran like a champ.
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  6. #26
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    Quote Originally Posted by K.L. Davis View Post
    Thanks... I was partial to a ~15.0 / 15.5 barrel for the middy gas system, the reason I say the 16" Middy is the best choice, is that it is not an NFA choice...

    The 14.5 middy does okay - lots of folks are running them, my personal opinion is that it is leaning on the short side of a barrel for a middy... but not so much I would ever discount it. I made up a barrel I called the "Aardvark" - it is an M4 with a mid-length gas system... the gas tube ran through the front sight to a gas block located in front of the gas block. I guess I like that setup, I still have one...



    But I think the barrel is 15" on it.

    The best 14.5 gun I have ever shot was one we did up with some guys in Ohio - it was for all intents and purposes a regular old M4, but the gas port was in the front "leg" of the FSB... about 1.5" forward of the carbine location... so about 0.5" short of the middy. It ran like a champ.
    What is your opinion on gas port sizes for 14.5" middies versus 16" middies? Do you rec a larger gp for the 14.5" middy as compared to the 16"? What size have you found works the best and provides the best "margin" for function?
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  7. #27
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    While not everyone's favorite configuration on M4C, I have a 18" w/rifle-gas. It has been tested and runs great in temps from -10 to 100F. Lube choice is important, some stuff tends to gum up more than others at low temps. I prefer ATF.

    M4C is a "carbine" oriented site, but given the adoption of the MK12 by the military, I believe it would be safe to say that 18" rifle-gas is a proven combination particularly if using a true 5.56 spec load like MK262.

    A much more sensitive caliber is 300 BLK, which makes sense given the expansion ratio of the cartridge.

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