NSWC testing on gas systems. CARBINE VS MID-LENGTH

[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):

[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.

[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.