20", .308, RIfle Gas, Gas Port Size? - Page 1

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[ARCHIVED THREAD] - 20", .308, RIfle Gas, Gas Port Size?

Posted: 4/11/2015 1:46:12 AM EDT

Help a guy out here?

What brand barrel do you have in 20" rifle gas .308 and what is the port size on it?

Thanks,

Posted: 4/11/2015 3:47:53 AM EDT

[#1]

If this helps any, the 1960's AR-10 (pg. 3) had a gas port diameter of 0.116", and a gas tube inside diameter measuring 0.134".

Posted: 4/11/2015 9:34:06 AM EDT

[#2]

Quote History

Quoted:
If this helps any, the 1960's AR-10 (pg. 3) had a gas port diameter of 0.116", and a gas tube inside diameter measuring 0.134".

View Quote

Thanks but wow that seems quite large. I have .089 for a .308, 20", rifle gas but am questioning if it seems standard to others.

I'm pretty sure DPMS is .089 just looking for feedback from others with potentially different manufacturers in the same system (.308, 20", rifle gas).

Posted: 4/11/2015 2:28:25 PM EDT

[#3]

I've been tuning my 20" gas port. I was having trouble with over pressurization. The port in the barrel was .159 and about the same in the gas block. I threaded the gas block with a 10-24 tread and installed a set screw that I drilled out until the bolt timing seemed to work reliably. I ended up with a .096 dia orifice. The bolt will lock and latch at .086 diameter but it it wouldn't work 100 % of the time when multiple cartridges in the magazine.

Posted: 4/11/2015 9:57:21 PM EDT

[#4]

Quote History

Quoted:

Thanks but wow that seems quite large. I have .089 for a .308, 20", rifle gas but am questioning if it seems standard to others.

I'm pretty sure DPMS is .089 just looking for feedback from others with potentially different manufacturers in the same system (.308, 20", rifle gas).

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Quote History

Quoted:

Quoted:
If this helps any, the 1960's AR-10 (pg. 3) had a gas port diameter of 0.116", and a gas tube inside diameter measuring 0.134".

Thanks but wow that seems quite large. I have .089 for a .308, 20", rifle gas but am questioning if it seems standard to others.

I'm pretty sure DPMS is .089 just looking for feedback from others with potentially different manufacturers in the same system (.308, 20", rifle gas).

Thinking it over some more, this AR-10 used an adjustable gas tube, as mentioned in this report. I would figure the barrel's gas port would be somewhat larger than the M-16's 0.093" gas port to account for the larger volume of the carrier's gas cylinder when compared to a M-16 carrier; and the larger volume of the AR-10 gas tube (0.134" AR-10 I.D. / 0.120" M-16 I.D.)

On a side note, are AR-10's and AR .308's still using the 0.134" gas tube inside diameter specification today? I'm wondering because the AR-15 gas tubes in my builds I've measured were 0.120".

Posted: 4/11/2015 11:08:51 PM EDT

[#5]

I've read that armalites measure at .084" .
My 18" 243 armalite runs fine at .077" it was a 24" T i cut down and reprofiled. Stock measure port was around .066"
Most 308 are way over gassed

Posted: 4/12/2015 7:26:12 AM EDT

[#6]

IIRC, the BHW .308 based barrels are in the .085" (DPMS Pattern) gas port diameter for rifle length gas systems...keep in mind
that will very with the diameter of the barrel at the journal/gas seat (I.E., the heavy bull barrels with say a .936" dia. gas port seat
will need a larger dia. gas port). Also, keep in mind that the ratio of gas volume to port dia. is heavily biased (what this means is
a small Increase in gas port dia. yields a large gain in gas volume/port pressure...) I don't recall the exact ratio, but it is something
like 10:1 .

Hope this helps...

Posted: 4/12/2015 3:27:07 PM EDT

[#7]

Quote History

Quoted:
If this helps any, the 1960's AR-10 (pg. 3) had a gas port diameter of 0.116", and a gas tube inside diameter measuring 0.134".

View Quote

Yeah, but the bolt and carrier was heavier.

Posted: 4/12/2015 3:49:15 PM EDT

[#8]

Quote History

Quoted:
Also, keep in mind that the ratio of gas volume to port dia. is heavily biased (what this means is a small Increase in gas port dia. yields a large gain in gas volume/port pressure...) I don't recall the exact ratio, but it is something like 10:1 .

Hope this helps...

View Quote

With a 0.117" ID tube, approximately 15 inches long (a standard rifle gas tube), going from a gas port of 0.062" to 0.092" increases the bolt carrier cavity pressure from 1,700 psi to 2,600 psi, going from 0.092" to 0.120" increases the cavity pressure to 4,000 psi.

That's basically a linear relationship between port increase to cavity pressure, a 53% increase in gas port diameter yields a 53% increase in cavity pressure.

Posted: 4/13/2015 11:29:24 AM EDT

[#9]

A Krieger 20" RLGS I just worked on with .936" journal had .0935" port, like an AR15. I sealed the gas block and tube, and it was gassed very hard.

A Krieger 6.5 Creedmoor has a .086" port, RLGS and .936" journal.

Bartlein uses .086" for 6.5 CM and .260 Rem as well.

.308 has more bore volume and case capacity than 5.56, so the ports actually match up more or less.

The 6.5-08 cases use slower-burning powders, with less bore volume, so port pressure is much higher, necessitating a smaller gas port to reduce violent extraction.

Posted: 4/13/2015 11:54:03 AM EDT

[#10]

Quote History

Quoted:
A Krieger 20" RLGS I just worked on with .936" journal had .0935" port, like an AR15. I sealed the gas block and tube, and it was gassed very hard.

A Krieger 6.5 Creedmoor has a .086" port, RLGS and .936" journal.

Bartlein uses .086" for 6.5 CM and .260 Rem as well.

.308 has more bore volume and case capacity than 5.56, so the ports actually match up more or less.

The 6.5-08 cases use slower-burning powders, with less bore volume, so port pressure is much higher, necessitating a smaller gas port to reduce violent extraction.

View Quote

Thanks. Doesn't sound like .089 is too far off with a .750 journal, rifle length, 20" .308 then.

Posted: 4/13/2015 12:00:05 PM EDT

[#11]

If you seal up a .089" gas port on a .750" RLGS .308, it should gas more than sufficiently.

Either clamp or Red Loc-tite the gas block in place after establishing alignment, and seal the gas tube into the block with Blue Loc-tite.

Sealing the gas block provides a very consistent set-up from day 1, where you're not worrying about carbon fouling changing the way the system gasses.

The large ports account for a lot of slop between gas block and barrel journals. KAC has addressed this with the Mod 2 system, which doesn't require any adhesives, but does require a different barrel profile with threads ahead of the journal for the nut.

Posted: 4/13/2015 12:10:29 PM EDT

[#12]

Quote History

Quoted:
If you seal up a .089" gas port on a .750" RLGS .308, it should gas more than sufficiently.

Either clamp or Red Loc-tite the gas block in place after establishing alignment, and seal the gas tube into the block with Blue Loc-tite.

Sealing the gas block provides a very consistent set-up from day 1, where you're not worrying about carbon fouling changing the way the system gasses.

The large ports account for a lot of slop between gas block and barrel journals. KAC has addressed this with the Mod 2 system, which doesn't require any adhesives, but does require a different barrel profile with threads ahead of the journal for the nut.

View Quote

I was just trying to see what people have out there... I've got a couple .308 barrels going on and have a couple 7-08 coming in, and want to play with the port sizes a bit. The 7-08 will end up smaller than the .308 no doubt but since there's no real industry standard to reference I figured a survey-type request/thread will do.

Most helpful, thanks.

[ARCHIVED THREAD] - 20", .308, RIfle Gas, Gas Port Size?

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