View Full Version : Extreme Cold Gas System Operation
This post by KL got me thinking on what effect cold temperatures have on the various gas system configurations.
This post implies that configurations work acceptably at normal temperatures,but fail at low temps, while others work in both.
I'd like to explore WHY this might be.
What experiences has anyone had with low temperature testing, freezing guns?
Are there old Army docs that cover this?
Lets stay away from cold weather LUBE consideration, as that's an orthogonal (unrelated) issue.
I've played around with gas port size and location quite a bit... you can get a 16" with Rifle Length system to run... but as I recall, all of the test guns were single shots when we froze them.
Short story is we did not spend a lot of time on the concept, because the 16" with Mid-Length will always be a better solution.
Simple physics. Part of the energy in the gas is in the heat. Heat flows to cold. Part of the energy is being siphoned off to heat the rifle. Chill the gas and it condenses and loses pressure
Regarding K.L's quote does that imply that they were function testing the guns in a sufficiently cold ambient environment or just sticking them in the freezer for a few hours and then testing function? dumb question I know....
Simple physics. Part of the energy in the gas is in the heat. Heat flows to cold. Part of the energy is being siphoned off to heat the rifle. Chill the gas and it condenses and loses pressure
You would have to have a gas system that was BARELY on the edge of functioning in order for the cooling effect to negate pressure fast enough to cause failures. Or you plan on shooting in liquid nitrogen...
Failures would also result from the chilling effect causing the metal parts to contract around one another. Eg: Gas Key / Gas Tube and Barrel / Bolt. And of course, lubricants freezing as well...
You would have to have a gas system that was BARELY on the edge of functioning in order for the cooling effect to negate pressure fast enough to cause failures...
Nevertheless, that's what happens. The colder the metal, the more heat it steals from the flame front reducing energy output. That's why your automobile runs less effeciently when the engine is cold. On top of that, when gunpowder itself is cold, it makes less pressure. The evidence is in the fact chilled ammo shows a drop in velocity
Failures would also result from the chilling effect causing the metal parts to contract around one another. Eg: Gas Key / Gas Tube and Barrel / Bolt. And of course, lubricants freezing as well...
Remember BOTH parts contract when cold and BOTH expand when hot. That's why the rifle is built with enough clearance to keep it from seizing up when hot or cold. The differences in expansion rates between the materials is part of what dictates clearances. Lubricants can thicken to the point they will slow the BCG, but that's taken into account when doing low temperature testing.
Just because it's rocket science, doesn't mean it's complicated
K.L. Davis
09-14-13, 16:42
Chest freezer, next to the shoot barrel... -10* F. I have to break the rifle down to put them in there. I consider it a mandatory part of any development testing.
Shoot rifle, do not clean
Break rifle down
Put upper, lower AND loaded magazine in the freezer
Some hours later (usually overnight) take out rifle, assemble, load and test fire
Yes, it makes a difference... of course the ammo being cold probably matters more than anything, and the type of lube is a factor; but I want to know what to expect in extreme conditions.
I was recently speaking with a guy from a gun company that is doing some ridiculous short barrel / long gas system combination that they "were able to make work" by hogging out the gas port and whatever other hacks needed... when I asked him if they were worried about it not functioning when it was cold, the answer was "that's one of the good things about living in Southern Arizona"
Might work for some, unacceptable for me... I don't want a fairweather rifle.
Regarding K.L's quote does that imply that they were function testing the guns in a sufficiently cold ambient environment or just sticking them in the freezer for a few hours and then testing function? dumb question I know....
While I have employed both techniques... I much prefer to sit inside and watch Cheap Grade-B Horror Movies while the rifle freezes its ass off by itself!
Do other configurations like the 16" midlength pass this test easily?
What other configurations fail? Is it primarily with the short dwell configurations?
Chest freezer, next to the shoot barrel... -10* F. I have to break the rifle down to put them in there. I consider it a mandatory part of any development testing.
Yes, it makes a difference... of course the ammo being cold probably matters more than anything, and the type of lube is a factor; but I want to know what to expect in extreme conditions.
I was recently speaking with a guy from a gun company that is doing some ridiculous short barrel / long gas system combination that they "were able to make work" by hogging out the gas port and whatever other hacks needed... when I asked him if they were worried about it not functioning when it was cold, the answer was "that's one of the good things about living in Southern Arizona"
Might work for some, unacceptable for me... I don't want a fairweather rifle.
While I have employed both techniques... I much prefer to sit inside and watch Cheap Grade-B Horror Movies while the rifle freezes its ass off by itself!
What kind of performance variation could one expect taking a weapon tuned at 70F down to teens? Should one consider switching to a lighter buffer for sub-freezing temps?
I'm thinking you may start to see short-stroking with weaker ammo that otherwise ran fine. I wouldn't expect any issues using NATO pressure ammo but there could be depending on how 'tuned' your weapon is at 70*. When I shoot in the winter time in 10*-20*F I notice reduced recoil with all types of ammo, at least until the weapon gets hot.
What kind of performance variation could one expect taking a weapon tuned at 70F down to teens? Should one consider switching to a lighter buffer for sub-freezing temps?
K.L. Davis
09-14-13, 19:48
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.
Do other configurations like the 16" midlength pass this test easily?
What other configurations fail? Is it primarily with the short dwell configurations?
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.
What kind of performance variation could one expect taking a weapon tuned at 70F down to teens? Should one consider switching to a lighter buffer for sub-freezing temps?
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.
crusader377
09-14-13, 22:17
What configurations do you think would pass the extremely cold test provided that they are a quality rifle and using standard 5.56 ammo ie M193 or M855.
I would venture the following four would definitely pass the test.
20" rifle
16" Midlength
16" carbine
14.5" carbine
What other should be added to the list?
Do other configurations like the 16" midlength pass this test easily? Clint, in real world testing at temps as low as -28F and cold sinks over over 24 hours, I have yet to have either my DD 16" middy or my BCM 16" middy fail in any way. It's not exactly high round count at those temps, but I haven't had an issue. This is with crappy .223 also, 5.56 would only perform better.
I've run my bcm middy in 100deg Texas heat at hogs and paper as well temps down to -40 deg above the arctic circle with the same h2 buffer and spring with absolutely no issues albeit a little sluggish.
I think all of those would pass too. I'd imagine 11.5 and 12.5 carbine would also work fine.
What configurations do you think would pass the extremely cold test provided that they are a quality rifle and using standard 5.56 ammo ie M193 or M855.
I would venture the following four would definitely pass the test.
20" rifle
16" Midlength
16" carbine
14.5" carbine
What other should be added to the list?
K.L. Davis
09-15-13, 00:39
What configurations do you think would pass the extremely cold test provided that they are a quality rifle and using standard 5.56 ammo ie M193 or M855.
I would venture the following four would definitely pass the test.
20" rifle
16" Midlength
16" carbine
14.5" carbine
What other should be added to the list?
As Eric D said, the 12.5 Carbine is for sure on the list... don't get too wound up about this, seriously; I do the testing on combinations as sort of a final acid test and there are variables between any two identical guns - even at that, most guns that are single shots, are only that way for one or two rounds...
Again, it is just a test I like to use to cull out systems that are "raged edge" of functioning... and as many people have pointed out the systems that are known to be reliable (standard rifle, 16" middy, M4 config, etc) are proven to work in even the most extreme spread of environments.
The ammo being cold will be a much larger factor IMO than the rifle. Many powders are highly temp dependent.
Suwannee Tim
09-17-13, 17:38
The Army and Marines have units trained to fight in arctic conditions, presumably they use standard weapons, M4s, M16s, everything else in those cold conditions. Isn't that right?
There's a general MILSPEC for all US military small arms and ammunition -- these are specifically for 5.56:
MIL-C-9963F
CARTRIDGE, 5.56MM, BALL M193
3.6 Velocity. The average velocity of the sample cartridges, conditioned at 70 + 2 Fahrenheit (F), shall be 3165 feet per second (ft/sec) plus or minus 40 ft/sec, at 78 feet from the muzzle of the weapon. The standard deviation of the velocities shall not exceed 40 ft/sec,
3.7 Chamber pressure.
3.7.1 Measurement by copper-crush cylinder. The average chamber pressure
o€ the sample cartridges, conditioned at 70 +/- 2F, shall not exceed 52,000 pounds per square inch (PSI). The average chamber pressure plus three standard deviations of chamber pressure shall not exceed 58,000 PSI.
3.7.2 Measurement by piezoelectric transducer. The average chamber pressure of the sample cartridges, conditioned at 70 + 2F, shall not exceed 55,000 PSI. The average chamber pressure plus three standard deviations of chamber pressure shall not exceed 61,000 PSI.
3.8 Port pressure.
3.8.1 Measurement by copper crush cylinder. The average port pressure of the sample cartridges, conditioned at 70 +/- 2F, shall be 15,000 PSI +/- 2000 PSI.
3.8.2 Measurement by piezoelectric transducer. The average port pressure of the sample cartridges, conditioned at 70 +/- 2F shall be shall be 14,400 PSI +/- 2000 PSI.
3.9 Temperature stability. When the sample cartridges are subjected to the following storage conditions, the average velocity shall not decrease by more than 250 ft/sec and the average chamber pressure by either method used in 3.7 shall not increase by more than 5000 PSI, the average port pressure by either method used in 3.8 shall neither increase nor decrease by more than 2000 PSI with respect to the average velocity, chamber pressure and port pressure of the sample cartridges of the same lot, conditioned at 70 +/- 2 F for a minimum of twenty minutes. Any increases in velocity and decreases in chamber pressure of the sample cartridges under these temperature conditions are acceptable,
Stored at 125 +/- 2F for not less than one hour and fired at that temperature.
Stored at -65 +/- 5F for not less than one hour and fired at that temperature.
末末末末末末末末末末末末末末末末末末末
MIL-C-63989C
CARTRIDGE, 5.56MM, BALL, M855
3.6 Velocity. The average velocity of the cartridges,
conditioned at 70 +/- 2F, shall be 3000 feet per second (fps) plus or minus 40 fps at 78 feet from the
muzzle of the weapon. The standard deviation of the velocities shall not exceed 40 fps.
3.7 Chamber pressure. The average chamber pressure of the sample cartridges, conditioned at 70 +/- 2F shall not exceed 55,000 psi. Neither the chamber pressure of an individual sample test cartridge nor the average chamber pressure plus three standard deviations of chamber pressure shall exceed 61,000 psi.
3.8 Port Pressure. The mean port pressure minus three
standard deviations shall not be less than 12,700 psi for sample cartridge conditioned to 70 +/- 2F.
3.10 Temperature stability. The action time, pressure and velocity of sample cartridges conditioned and fired at the temperature extremes specified below shall be in accordance with 3.10.1, 3.10.2, 3.10.3, and 3.10.4
a. Conditioned at 125 +/- 2F for not less than one hour and fired at that temperature.
b. Conditioned at -65 +/- 2F for not less than one hour and fired at that temperature.
There's a general MILSPEC for all US military small arms and ammunition -- these are specifically for 5.56:
-2000 psi port pressure or about 85% of normal
No spec for chamber pressure loss.
It looks like ammo "power" can be expected to be down by at least 10% in cold environments.
even at that, most guns that are single shots, are only that way for one or two rounds...
and as many people have pointed out the systems that are known to be reliable (standard rifle, 16" middy, M4 config, etc) are proven to work in even the most extreme spread of environments.
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.
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.
Alaskapopo
09-19-13, 23:26
Use thinner lube. A friend of mine working in temps as low as -50 uses 0 weight motor oil and it works great. Also avoid having the gun go inside and outside in these conditions. Condensation turns to ice and can stop the guns cold.
Pat
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.
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.
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.
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
BufordTJustice
09-21-13, 12:32
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?
K.L. Davis
09-22-13, 01:03
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...
https://www.m4carbine.net/picture.php?albumid=687&pictureid=3402
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.
BufordTJustice
09-29-13, 17:28
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...
https://www.m4carbine.net/picture.php?albumid=687&pictureid=3402
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?
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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