NSWC testing on gas systems. CARBINE VS MID-LENGTH

[npena84]

So at the NDIA conference a study was revealed. That tested the reliability, durability, and accuracy (among others). Of the mid-length gas system vs the carbine length. It seems to confirm much of what is already known. With a couple of surprises throw in as well. Worth the read.

http://soldiersystems.net/2018/05/14/nswc-crane-carbine-mid-length-gas-system-testing-shows-increased-performance/

[Serious Account]

Very interesting. So it's been confirmed that midlength is indeed superior.

I wonder what gas port size are they using for the mid, and how well will it function with weaker/underpowered ammo + H2 buffer. Are they optimizing this for M855A1 only?

My guess is 14 in mid is likely to be a little more finicky with ammo than carbine gas.

[Mr. Goodtimes]

I’m curious how accuracy degradation started at a mere 7,000 rounds in the carbine length guns. That’s not very many rounds. Also the test is skewed in terms of barrel life as the gas system has nothing to do with that and the mid length barrels are hammer forged where as the carbine ones are not.


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[Coal Dragger]

Good read, although I wonder if the barrel accuracy life findings have more to do with cold hammer forging than where the gas port is located?

[Mr. Goodtimes]

Good read, although I wonder if the barrel accuracy life findings have more to do with cold hammer forging than where the gas port is located?

They definitely do, as gas port location has nothing to do with barrel life, however, I’m curious to know how they were only getting 7k rds is barrel life out of a regular barrel.


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[npena84]

Could the barrel degradation be the results of using EPR?

[alx01]

I’m curious how accuracy degradation started at a mere 7,000 rounds in the carbine length guns. That’s not very many rounds. Also the test is skewed in terms of barrel life as the gas system has nothing to do with that and the mid length barrels are hammer forged where as the carbine ones are not.


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Am I mistaken or did I read somewhere on m4c that COLT non-hammer forged barrels lasted over 60k rounds? I simply don't believe that regular COLT barrels degrade at 7k rounds. Maybe SBR's like MK18 at longer distances.

In my anecdotal experience, typically it's almost impossible to tell the accuracy degradation if doing groups at 100m. As barrels age they still tend to group well at 100-200m (even 250m). Once you start going to 400-600m range is when you can really tell the difference between a good barrel with 10k barrel rounds and a worn/bad barrel. Too many factors affect the barrel life in my opinion including cleaning schedule, production methods/materials, most importantly rate of fire the barrel has been through. This is true regardless whenever the barrel is CHF or not.

I find it a bit amusing that people think that CHF is always superior - its only better if process, materials, and treatment are suitable. Yet, there are millions upon millions of non-CHF barrels that perform fine. I suspect, in a regular use scenarios (mil or civ) it's impossible to tell the difference between the two types besides the markings.

Would I choose a CHF barrel over the regular one? In case of BCM, Noveske, DD, or other known manufacturer - probably yes. But I wouldn't be hell bent on it. By the same logic, I think it's always better to choose a regular barrel of a well known brand over CHF of questionable production.

[Bourne911]

That accuracy standard is pretty crap .
http://gshort.click/buluhidung/79/o.png

[Glock9mm1990]

This is interesting to say the least, but only 6000 rounds in a Colt barrel? That must be because of the M855A1 round.

[ABNAK]

Yeah I'm thinking M855A1 has something to do with that.

Also, how do you get a velocity difference at 100m? i.e. how is that impacted if the muzzle velocities are similar? Gun X MV is 2900fps. Gun Y MV is 2900 fps. Yet Gun X is going faster at 100m? How the hell does that happen using the same ammo?

[Coal Dragger]

Maybe the faster gun at 100M is achieving better stabilization of the round resulting in less oblong rotation of the bullet and better aerodynamic performance?

[MegademiC]

Maybe the faster gun at 100M is achieving better stabilization of the round resulting in less oblong rotation of the bullet and better aerodynamic performance?

I was thinking that could contribute to velocity as well. They are pretty much identical at muzzle, but the mid slows down less.

I dont get the cyclic rate at all . Can anyone elaborate on that? The concept makes sense, but the numbers dont- cyclic rate if 1rd per second?

[vicious_cb]

I was thinking that could contribute to velocity as well. They are pretty much identical at muzzle, but the mid slows down less.

I dont get the cyclic rate at all . Can anyone elaborate on that? The concept makes sense, but the numbers dont- cyclic rate if 1rd per second?


Maybe the faster gun at 100M is achieving better stabilization of the round resulting in less oblong rotation of the bullet and better aerodynamic performance?

That doesnt make sense either, same twist rate, same barrel length should have the same stability.

Whats wrong with the cyclic rate figures? They fit perfectly in line with lower port pressures=lower cyclic rate suppressed and unsuppressed.

[Serious Account]

A bit unrelated but.. does port erosion count as well when it comes to barrel lifespan?

CHF barrel combine with midlength gas could mean significant lifespan for the barrel and gasport.

[EzGoingKev]

Also, how do you get a velocity difference at 100m? i.e. how is that impacted if the muzzle velocities are similar? Gun X MV is 2900fps. Gun Y MV is 2900 fps. Yet Gun X is going faster at 100m? How the hell does that happen using the same ammo?
Thinking about how things work, the rifle with the carbine length gas system is going to unlock the bolt earlier than the mid-length. I am thinking that would yield an increase in muzzle velocity.

[vicious_cb]

Thinking about how things work, the rifle with the carbine length gas system is going to unlock the bolt earlier than the mid-length. I am thinking that would yield an increase in muzzle velocity.

Then you would see an increase in both muzzle velocity and 100 yard velocity.

[Mr. Goodtimes]

That accuracy standard is pretty crap .
http://gshort.click/buluhidung/79/o.png

It’s mostly due to the crap ammo they’re probably feeding the weapons.


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[npena84]

I'm thinking that CHF barrel, that mid length gas, and that EPR round. Sound like a nice little combo, especially if you can get Joe proof 1-6.

[MegademiC]

That doesnt make sense either, same twist rate, same barrel length should have the same stability.

Whats wrong with the cyclic rate figures? They fit perfectly in line with lower port pressures=lower cyclic rate suppressed and unsuppressed.

Edit- I read a 62 round cyclic rate, not 62 rpm less. I miss-read it.

[Coal Dragger]

That doesnt make sense either, same twist rate, same barrel length should have the same stability.

Whats wrong with the cyclic rate figures? They fit perfectly in line with lower port pressures=lower cyclic rate suppressed and unsuppressed.

Not necessarily. All things being equal stability should be the same, but some barrels are more accurate than others. Why is that? Might be something to do with the chamber, and lead being more concentric to the bore, allowing the bullet to enter the rifling lined up perfectly. Hammer forged barrels usually feature a chamber that is forged along with the rifling, everything is concentric.

There is a phenomenon where sometimes a bullet doesn’t “go to sleep” right away and will display some yaw on the rotational axis. It’s been noted mostly with very high velocity projectiles. I read an article once about a Weatherby long range rifle in .30-378 Wby that was nothing special at 100 yards, but the same gun with the same ammo shot the exact same size group at 300 yards that it did at 100 yards. It took awhile for the bullet to settle into a perfect spiral axis.

If the CHF barrels are that much more accurate in testing with the same ammo, it tells me that the CHF barrel is helping that projectile be more consistent in some way.

[Clint]

14.5 MID is another "optimum" configuration.

The comments say they use a .076" gas port, which puts the gas drive right at the "5.56" level.

Looks like they nailed it.

[vicious_cb]

14.5 MID is another "optimum" configuration.

The comments say they use a .076" gas port, which puts the gas drive right at the "5.56" level.

Looks like they nailed it.

Nah, for some odd reason they decided to stick with gov't profile. They had to screw something up, it is gov procurement after all :rolleyes:

[mack7.62]

You know from the original testing 13" was the optimum length for carbine gas, Marines wanted to mount bayonets so we get 14.5, so this all makes sense. This testing has answered a question I had about 14.5 mid cold weather so nice to know I no longer have to wait for a -40 day and then go shooting. :D

[MegademiC]

I aleays assumed the carbine gas was designed around a 10” barrel on the xm177.

[hile]

I aleays assumed the carbine gas was designed around a 10” barrel on the xm177.

I thought the same thing. I use mid length on my 14.5s and 16s preferentially.

[Clint]

In '67, Colt lengthened the xm177 to 11.5 inches

[sinister]

Accuracy starts to degrade quicker on the 7" carbine gas length as the downrange side of the gas port erodes quicker. You're talking a significant port pressure delta in those two inches' difference between 7 and 9 inches.

[Diamondback]

Forgive me if I missed this, but did anybody ever have a table of optimal gas lengths for various barrels and chamberings? Sorry, my memory's just not what it used to be...

[ABNAK]

That doesnt make sense either, same twist rate, same barrel length should have the same stability.

Whats wrong with the cyclic rate figures? They fit perfectly in line with lower port pressures=lower cyclic rate suppressed and unsuppressed.

I considered that but, just as you said, it doesn't make sense.

[ABNAK]

Not necessarily. All things being equal stability should be the same, but some barrels are more accurate than others. Why is that? Might be something to do with the chamber, and lead being more concentric to the bore, allowing the bullet to enter the rifling lined up perfectly. Hammer forged barrels usually feature a chamber that is forged along with the rifling, everything is concentric.

There is a phenomenon where sometimes a bullet doesn’t “go to sleep” right away and will display some yaw on the rotational axis. It’s been noted mostly with very high velocity projectiles. I read an article once about a Weatherby long range rifle in .30-378 Wby that was nothing special at 100 yards, but the same gun with the same ammo shot the exact same size group at 300 yards that it did at 100 yards. It took awhile for the bullet to settle into a perfect spiral axis.

If the CHF barrels are that much more accurate in testing with the same ammo, it tells me that the CHF barrel is helping that projectile be more consistent in some way.

That MIGHT, maybe, be the only answer that makes sense. i.e. it's CHF vs non-CHF. Maybe......

EDIT: *might* affect accuracy but not velocity.

Wonder who made the CHF barrels for this test?

[markm]

Accuracy starts to degrade quicker on the 7" carbine gas length as the downrange side of the gas port erodes quicker. You're talking a significant port pressure delta in those two inches' difference between 7 and 9 inches.

I used to have those charts... out of quick load. They'd show the pressure levels throughout the barrel... 3-5k reduction from carbine to mid if I remember right.

[sinister]

Armalite had a very good pressure curve graphic posted on their tech notes. This is a variation (63-grains being close to legacy M855 Green Tip):

http://www.ar15barrels.com/gfx/223plot.gif
http://www.guntweaks.com/uploads/1/9/6/5/19655623/ar-gas-gystem-port-pressure_1_orig.png

[Coal Dragger]

That MIGHT, maybe, be the only answer that makes sense. i.e. it's CHF vs non-CHF. Maybe......

EDIT: *might* affect accuracy but not velocity.

Wonder who made the CHF barrels for this test?

If you have a projectile that enters the rifling at an non optimal angle due to a less than concentric chamber, muzzle velocity won’t be affected but downrange performance of that ever so slightly deformed round with an ever so slight oblong movement to its spiral rotation will show degraded accuracy and probably velocity.

[Clint]

Forgive me if I missed this, but did anybody ever have a table of optimal gas lengths for various barrels and chamberings? Sorry, my memory's just not what it used to be...

Here is a good start.

https://dqzrr9k4bjpzk.cloudfront.net/images/812059/855764506.jpg

[lysander]

They definitely do, as gas port location has nothing to do with barrel life, however, I’m curious to know how they were only getting 7k rds is barrel life out of a regular barrel.


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Full auto-mag dumps.....


Yeah I'm thinking M855A1 has something to do with that.

Also, how do you get a velocity difference at 100m? i.e. how is that impacted if the muzzle velocities are similar? Gun X MV is 2900fps. Gun Y MV is 2900 fps. Yet Gun X is going faster at 100m? How the hell does that happen using the same ammo?
They aren't getting higher velocities at 100 m, that 30 to 40 fps is well within the SD of that ammo.

The big take-away of that article is:

1) Cyclic rate (suppressed/unsuppressed):
--carbine: 944/864 rpm
--mid: 881/737 rpm

2) MRBF:
Ambient
--carbine: 836.1 (65 malfunctions)
--mid: 1993.8 (30 malfunctions)

The two are related, BTW.

[Clint]

The big take-away of that article is:

1) Cyclic rate (suppressed/unsuppressed):
--carbine: 944/864 rpm
--mid: 881/737 rpm

2) MRBF:
Ambient
--carbine: 836.1 (65 malfunctions)
--mid: 1993.8 (30 malfunctions)

The two are related, BTW.

True.

Interesting ( but completely expected ) was the % cyclic rate increase when suppressed
Carbine +9%
Mid +19%

Is that carbine cyclic rate (864 rpm) typical of an M4 with mil-spec gas port? Seems a little higher than optimal.

[redpillregret]

Not necessarily. All things being equal stability should be the same, but some barrels are more accurate than others. Why is that? Might be something to do with the chamber, and lead being more concentric to the bore, allowing the bullet to enter the rifling lined up perfectly. Hammer forged barrels usually feature a chamber that is forged along with the rifling, everything is concentric.

There is a phenomenon where sometimes a bullet doesn’t “go to sleep” right away and will display some yaw on the rotational axis. It’s been noted mostly with very high velocity projectiles. I read an article once about a Weatherby long range rifle in .30-378 Wby that was nothing special at 100 yards, but the same gun with the same ammo shot the exact same size group at 300 yards that it did at 100 yards. It took awhile for the bullet to settle into a perfect spiral axis.

If the CHF barrels are that much more accurate in testing with the same ammo, it tells me that the CHF barrel is helping that projectile be more consistent in some way.



Do you have any links to a credible ballistician that can confirm the phenomenon you describe? I’d like to see confirmation of this often repeated theory.

From what I’ve read, the bullet will continue forward from the point which it reaches stability; not returning to a central and consistent point along the rotational axis; ie making a consistent point of impact and thus small group. Typically the smaller groups are often related to shooter performance and buckling down.


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[Coal Dragger]

Sadly no, just memory of a Guns and Ammo article from years ago.

[lysander]

True.

Interesting ( but completely expected ) was the % cyclic rate increase when suppressed
Carbine +9%
Mid +19%

Is that carbine cyclic rate (864 rpm) typical of an M4 with mil-spec gas port? Seems a little higher than optimal.

In a report from Picatinny, 15 new M4A1 were tested for something else, but the cyclic rates were given.

W1-1: 822
W1-2: 859
W1-3: 794
W2-1: 804
W2-2: 735
W2-3: 832
W3-1: 820
W3-2: 781
W3-3: 747
W4-1: 712
W4-2: 656
W4-3: 773
W5-1: 754
W5-2: 788
W5-3: 806

Average: 788

Another report gives the average cyclic rate for ten M16A3s as 686.

The specification requirement for M4A1 Carbines is "...700 to 1025 rounds per minute when firing M855..."

So, the given cyclic rates seems reasonable.

[WS6]

14.5 MID is another "optimum" configuration.

The comments say they use a .076" gas port, which puts the gas drive right at the "5.56" level.

Looks like they nailed it.

That's what they allege...even if the barrel is daniel defense, who knows the real port size. They have done different ports for civilian and military for years.