NSWC testing on gas systems. CARBINE VS MID-LENGTH

[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

[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.

[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?