Barrel Gas Port Size,308/338/260/243 etc.

[308FAN]

10 hours ago, 98Z5V said:

 

Needs a 0.090" gas port diameter - or very, very damn close to that.

Outstanding! Thanks, for the help. I was going post my troubles in another category.  I didn't want to clog up an info thread.

[Leaflike92]

Criterion Barrel, SS Nitrided, .308, 16" barrel, rifle length gas, 15.25" tube, .094 gas port that is ~3.03" inches from center of hole to end of the barrel. Wanting to reliably run m80 ball. First post.

[98Z5V]

51 minutes ago, Leaflike92 said:

Criterion Barrel, SS Nitrided, .308, 16" barrel, rifle length gas, 15.25" tube, .094 gas port that is ~3.03" inches from center of hole to end of the barrel. Wanting to reliably run m80 ball. First post.

What is your gas block journal diameter?  What size gas block is needed?  Doubt it's 0.625", but it could be 0.750", 0.875", or 0.936".

Back in the day, when it was Kreiger/Criterion, they made that profile barrel custom for Fulton Armory, it used a 0.936" gas block journal, because it needed a 0.105" gas port.

[Leaflike92]

It's .750 diameter. Was planning on putting a Wojtek click adjustable block on.

Edited May 22, 2022 by Leaflike92

[98Z5V]

7 minutes ago, Leaflike92 said:

It's .750 diameter. Was planning on putting a Wojtek click adjustable block on.

You can try that gas port size out, but I'm pretty positive that it's going to be too small, and you'll need to go up.

Start a new thread on the build section, on your gun specifics, and go from there.  No sense cluttering this thread up with the data, that may or may not be useful to this thread in the future.

Edited May 22, 2022 by 98Z5V

[Leaflike92]

1 minute ago, 98Z5V said:

You can try that gas port size out, but I'm pretty positive that it's going to be too small, and you'll need to go up.

Thank you for the response, I've been reading though the forums a lot recently and it's clear to me that you are the community expert when it comes to things like this. I had the same hunch that it would likely be too small based on what I've read about manufacturers drilling these ports too small.

 

[Dr. LeVar Burstin]

16” Douglas 308WIN barrel. KAC E3 gas system (13.125” gas tube) .750” journal. Had Compass Lake drill a .086 gas port. Should be slightly over gassed? Running a Superlative AGB. Haven’t shot it yet. Kinda waiting on the AEM5-30 to clear. 

[98Z5V]

8 minutes ago, Dr. LeVar Burstin said:

Kinda waiting on the AEM5-30 to clear. 

Interesting that you got one - how long has it been on order?

[Dr. LeVar Burstin]

Pre-ordered the AEM5-30 thru Capitol Armory late March of 2021. Cleared F3 late January of this year, been pending since early February. It’s the chubby guy on the left:

Edited July 1, 2022 by Dr. LeVar Burstin

[SolidBuilt]

LR-308 16" Wilson combat recon tactical fluted match grade barrel with .750 journal and 13.25"" mid-length gas tube.

Factory 5/64" gas port. (.078)

Worked great with Yankee Hill Machine Flash Hider. 

Short stroked when I installed the "lil bastard" AMA muzzle brake .

Drilled gas port bigger with 3/32" bit. (.094)

Tons of gas now... maybe too much... but, it cycles great... plan on installing heavier buffer weight (5.3 oz) and orange Sprinco spring. 

I also run a Superlative arms adjustable gas block with it...

@98Z5V ??

Edited November 13, 2022 by SolidBuilt
Add info

[Ws318]

I have a 6.5 creedmoor with a 20 inch heavy barrel (.936 gas block) with a .093 gas port, and roughly 6.8 inches from the end of the barrel, but it will not cycle right. It barely ejects the casing when it does, most of the time it stove pipes them. With the heavier barrel do i need a larger gas port? Or an adjustable gas block? I’ve tried 3 different types of ammo and none run better than the other. 

[shooterrex]

Is this a new gun?

Is the Bolt carrier group dripping w/oil?

How many rounds through the gun?

Please do a search for the "waterboarding thread" and answer those questions.

Hard to fix something over the internet without a lot of exact details.

[Ws318]

23 minutes ago, shooterrex said:

Is this a new gun?

Is the Bolt carrier group dripping w/oil?

How many rounds through the gun?

Please do a search for the "waterboarding thread" and answer those questions.

Hard to fix something over the internet without a lot of exact details.

Yes. Its a new bear creek side charge upper. I cleaned it before it was ever shot and lightly lubed it so it wasn’t dripping oil. Its had 60 rounds though it at the moment. I think its only ejected 3-4 of them. And occasionally will lock back after firing without completely ejecting the case. I started only loading 1 at a time to keep it from jamming as bad. I looked at the waterboarding thread. The lower is a palmetto state with magpul grip and stock so im not sure what springs they use, but I’ll get the lengths and weights for the other parts, and a few pics as soon as i can. 

[98Z5V]

9 hours ago, Ws318 said:

I have a 6.5 creedmoor with a 20 inch heavy barrel (.936 gas block) with a .093 gas port, and roughly 6.8 inches from the end of the barrel, but it will not cycle right. It barely ejects the casing when it does, most of the time it stove pipes them. With the heavier barrel do i need a larger gas port? Or an adjustable gas block? I’ve tried 3 different types of ammo and none run better than the other. 

For that barrel profile and cartridge, you could use a gas port diameter of 0.070".  Your gas port is WAY oversized.  We can't shrink them, so you'll have to put an adjustable gas block on there.

My references for that are 20" rifle gas .260 Rem with a 0.750" gas block journal size, and it needed an 0.080" gas port. 

That's also on a balanced recoil system, so you'll need to very receiver extension internal depth, buffer length and weight.  We'll make sure you know what the proper spring is for those dimensions you give.  It's either gonna be the Sprinco Orange spring, or Armalite EA-1095 spring (or Sprinco Red spring). 

Buffer weight needs to be in the 5.4oz area - damn close. 

[MarkLeupold]

New info: 

Lilja 16.25" .308 Win barrel. This barrel has a 1-8" twist, a .750" gas block journal, and a .089" gas port.

JP FMOS BCG + LAW folder dingus

H3 buffer (3-1/4" length) 

Tubb's AR-10 flat wire spring

A5 buffer tube (7-5/8" depth)

 

Runs like a champ. Maybe slightly overgassed if 2:30 ejection means anything on the large frame rifles, but it shot extremely smooth today. 

[98Z5V]

6 hours ago, MarkLeupold said:

New info: 

Lilja 16.25" .308 Win barrel. This barrel has a 1-8" twist, a .750" gas block journal, and a .089" gas port.

JP FMOS BCG + LAW folder dingus

H3 buffer (3-1/4" length) 

Tubb's AR-10 flat wire spring

A5 buffer tube (7-5/8" depth)

 

Runs like a champ. Maybe slightly overgassed if 2:30 ejection means anything on the large frame rifles, but it shot extremely smooth today. 

That's amazing - but you failed to mention what the gas system length was, entirely.  State the gas system type (on a .308 AR it's either gonna be midlength gas or rifle gas), and the length of your gas tube, to make this post relevant for anyone to use in the future.  Otherwise - worthless. 

Most pertinent detail ever, in determining gas port sizing - gas system length. 

Missing here.

[MarkLeupold]

 

11 hours ago, 98Z5V said:

That's amazing - but you failed to mention what the gas system length was, entirely.  State the gas system type (on a .308 AR it's either gonna be midlength gas or rifle gas), and the length of your gas tube, to make this post relevant for anyone to use in the future.  Otherwise - worthless. 

Most pertinent detail ever, in determining gas port sizing - gas system length. 

Missing here.

18 hours ago, MarkLeupold said:

New info: 

Lilja 16.25" .308 Win barrel. This barrel has a 1-8" twist, a .750" gas block journal, and a .089" gas port.

JP FMOS BCG + LAW folder dingus

H3 buffer (3-1/4" length) 

Tubb's AR-10 flat wire spring

A5 buffer tube (7-5/8" depth)

 

Runs like a champ. Maybe slightly overgassed if 2:30 ejection means anything on the large frame rifles, but it shot extremely smooth today. 

Yep, you're right. Bonehead move. It's a midlength gas system. 11-3/4" gas tube

[98Z5V]

43 minutes ago, MarkLeupold said:

 

Yep, you're right. Bonehead move. It's a midlength gas system. 11-3/4" gas tube

That's an AR15 Midlength gas tube - got a pic of where it ends in the cam pin cutout in the upper receiver?

[MarkLeupold]

8 minutes ago, 98Z5V said:

That's an AR15 Midlength gas tube - got a pic of where it ends in the cam pin cutout in the upper receiver?

Of course.

[98Z5V]

That's perfect - that means Lilja is drilling the AR15 gas port location.  Good info. 

[98Z5V]

Wilson Combat Tactical Hunter barrel in 7mm-08 caliber.  18" Rifle Gas, 0.750" gas block journal diameter.  Ships with a gas port that 0.076".

I'll check some math and see if this is gonna work. 

[98Z5V]

On 1/22/2023 at 1:29 PM, 98Z5V said:

Wilson Combat Tactical Hunter barrel in 7mm-08 caliber.  18" Rifle Gas, 0.750" gas block journal diameter.  Ships with a gas port that 0.076".

I'll check some math and see if this is gonna work. 

No possible way this 0.076" gas port will work.  Will never happen, with a proper recoil system in place.  It's gonna have to be 0.085" minimum, and probably 0.090" in the end.  I think it's just gonna get the 0.090" treatment right away.  As it is, it's just not right. 

Thanks again, WC. 

[98Z5V]

Determinations.  Cylinder volume on the .260 Rem is 0.0012214.  That's a 20" rifle gas barrel with a 0.750" journal and the 0.080" gas port.  That's running a 2.500" 5.15oz buffer, lightest that I've tried to use that works.  That's not the proper buffer weight, not 5.4oz, so that port diameter might need to be slightly larger to run a 5.4oz proper buffer.  That gun was an experiment (the ONLY time, ever) that I've ever run a 2.500" buffer in a large frame, carbine recoil system.  No other large frames I've built use that, run that - and it was a proof of concept, and it works.  That was as heavy as I could get that little stubby 2.500" buffer in 2018. That barrel shipped with a 0.070" port, and had problems.  I had to figure out why. 

That number on the .260 gas port came from the common 18" Rifle gas barrels in .308 Win, 0.750" journals.  Those need 0.095" to run right, minimum, sometimes a little more.  Backwards planning had me go to the 0.080" on that .260 Rem.  It runs like a champ, is laser-accurate, and just works. No need to change it.  My 2.500" buffer-use experiment is done.  I'll never use another one, though, and haven't ever.

On this 18" .308 Win rifle gas barrel, 0.750 journal, at 0.095" port, the cylinder volume is 0.0015644. 

On this 7mm-08 barrel, as shipped, 0.750 journal, 18" rifle gas, that comes to a cylinder volume of 0.0010569.  Not gonna do it.  Will have problems in operation.  If I increase the gas port to a 0.090", cylinder volume goes to 0.0014822.  That will work, and it's pretty close to 18" rifle gas 0.750 journal .308 Win numbers. 

This is all nugged out with a proper recoil system, with a buffer weight of 5.4oz.  And a proper recoil spring for whatever extension you're using.  If THAT PART isn't right, none of these numbers mean shiit.  This is balancing the proper recoil system, and then balancing a gas system that matches it.  This is what makes guns reliable. 

Weaponize math, brothers.    That's what it's all about.  Math.  It's fuckin' Nerd Magic.

Edited January 28, 2023 by 98Z5V

[Albroswift]

Faxon 300BLK 16" Carbine gas .750 .120

Faxon 5.56 16" pencil w/ intregal 3 port comp mid gas .625 .079

Don't ask me how I know...

[MarkLeupold]

On 1/27/2023 at 11:17 PM, 98Z5V said:

Determinations.  Cylinder volume on the .260 Rem is 0.0012214.  That's a 20" rifle gas barrel with a 0.750" journal and the 0.080" gas port.  That's running a 2.500" 5.15oz buffer, lightest that I've tried to use that works.  That's not the proper buffer weight, not 5.4oz, so that port diameter might need to be slightly larger to run a 5.4oz proper buffer.  That gun was an experiment (the ONLY time, ever) that I've ever run a 2.500" buffer in a large frame, carbine recoil system.  No other large frames I've built use that, run that - and it was a proof of concept, and it works.  That was as heavy as I could get that little stubby 2.500" buffer in 2018. That barrel shipped with a 0.070" port, and had problems.  I had to figure out why. 

That number on the .260 gas port came from the common 18" Rifle gas barrels in .308 Win, 0.750" journals.  Those need 0.095" to run right, minimum, sometimes a little more.  Backwards planning had me go to the 0.080" on that .260 Rem.  It runs like a champ, is laser-accurate, and just works. No need to change it.  My 2.500" buffer-use experiment is done.  I'll never use another one, though, and haven't ever.

On this 18" .308 Win rifle gas barrel, 0.750 journal, at 0.095" port, the cylinder volume is 0.0015644. 

On this 7mm-08 barrel, as shipped, 0.750 journal, 18" rifle gas, that comes to a cylinder volume of 0.0010569.  Not gonna do it.  Will have problems in operation.  If I increase the gas port to a 0.090", cylinder volume goes to 0.0014822.  That will work, and it's pretty close to 18" rifle gas 0.750 journal .308 Win numbers. 

This is all nugged out with a proper recoil system, with a buffer weight of 5.4oz.  And a proper recoil spring for whatever extension you're using.  If THAT PART isn't right, none of these numbers mean shiit.  This is balancing the proper recoil system, and then balancing a gas system that matches it.  This is what makes guns reliable. 

Weaponize math, brothers.    That's what it's all about.  Math.  It's fuckin' Nerd Magic.

I would love to learn more about how you run these numbers and what you're looking for when you do. Is the cylinder volume the total volume of the barrel and gas system to the gas port?