Manufacturer Gas Port Dimensions | Primary & Secondary

Manufacturer Gas Port Dimensions

Discussion in 'Rifle Assembly' started by Jackrabbit, Jul 1, 2016.

  1. Jackrabbit

    Jackrabbit Member

    After taking the Semper Paratus AR-15 Armorer's course last weekend, I started wondering about what gas port sizes different barrel manufacturers use, especially in the context of building rifles or diagnosing gas issues.

    This wound up being an interesting side project for lunch breaks at work, as I spoke on the phone or via email with representatives from 17 different companies who manufacture their own barrels, and discussing the topic of gas ports on the Facebook side of the house. I figured out fairly early that if I phrased the question in the context of building a rifle with the company's product, I got better responses than if I just flat out asked the question.

    I suspect that what I was told on the phone by reps and what a pin gauge set would tell me about these barrels would not always be the same spec, especially with certain companies who have documented QA/QC issues and who are more interested in selling guns than providing a top quality product. This suspicion is further confirmed by Michael from Sons of Liberty Gun Works, who posted a photo of actual gas port dimensions which have come through his shop--some of the variances were huge, even between barrels from the same manufacturer. The next step in compiling information on gas ports would be to start actually using a pin gauge on barrels and recording the results, and comparing those measurements to what I was told by representatives of these companies.

    Here are some common themes I saw this week:
    -Rep thinking I was talking about the diameter of the barrel at the gas block
    -Once that issue was cleared up, being transferred to a desk where I left a message and was called back
    -Tech acting very puzzled about why I would be asking him this question.

    Some highlights:
    -Gene with LMT saw my initial post on Novice/Intermediate, so when I called him, he knew what I was up to. I was used to catching techs off guard with the question, instead he caught me off guard with his answer.
    -Michael with SOLGW posting the entire list of gas port dimensions on Novice/Secondary, with the caption "We ain't scared."
    -"Izzy" with Invincible Arms (who studied under Eugene "Gene" Stoner, according to him) acting very, very suspicious about why I would be asking him this question. I spoke with him for about 10 minutes about ammunition differences and gas port diameter before my lunch break ended.
    -Josh at Criterion handling an odd situation with grace, as he needed permission from Midwest Industries to release the gas port dimensions to some random knucklehead on the phone.
    -BCM responding to my email with a statement saying their gas port size is proprietary. I happen to have a 14.5M barrel sitting on my desk--the gas port is .076". It boggles me that something so easily confirmed would be considered proprietary.
    -No, I don't know that. I don't think anyone knows that." -Sig Sauer representative.

    Attached Files:

  2. Jackrabbit

    Jackrabbit Member

    One correction: Sionics does not make a 14.5M barrel, and their gas port on the 11.5 is .076
    Tactical22 likes this.
  3. Tactical22

    Tactical22 Newbie

    Sionics looks to be critically over-gassed. :p
  4. When teaching Advanced AR15 / M16 Armorer courses, we measure gas ports on all the barrels that students bring to class, and find a huge variation, and compare what they brought to the decades of collection gas port data that we have measured and collected. Generally we find that most carbine and rifle length systems are within a window, and mid-length is all over the place. We also let students use a borescope and look at the inside of the barrel, getting a close up magnified view of the gas port erosion, rifling, fouling, muzzle erosion, crown, chamber (throat, lead, free-bore). Students find that barrels erode from use, and cleaning the wrong way also causes damage. There is a difference in erosion on chrome lined barrels that have their gas port drilled before the chrome is applied, versus drilled after chrome is applied, which is why we drill gas ports before chrome lining barrels. Then we discuss how barrel materials, machining procedures, finishing, treatments, ammunition & shooting, erosion, cleaning products & methods, all effect barrel performance and life, and then we compare it to the data that is reference in that class manuals. This hands on approach allows students to make an educated decision of what to look for in choosing a barrel, how to properly maintain a barrel, ammunition choices and effects, shooting issues, and most of all troubleshooting and knowing when a barrel is worn out.

    Greg Sullivan "Sully"
    SLR15 Rifles
    (763) 712-0123
  5. Andrew Y.

    Andrew Y. Regular Member

    So people say that Daniel Defense is over gassed, yet their gas port on the 16M is smaller than most over brands, can some one get me up to speed on the disconnect?
  6. Arete

    Arete Member

    Not sure about DD's 11.5 and longer barrels, but DD's 10.3" barrels are widely reputed to be 0.083" until the past few years. The more recent ones are about 0.078". I've not measured one myself, but have seen numerous references to this over the years, including people stating that was what DD told them.

    The Crane spec for the Mk18 GP is 0.070".
    Andrew Y. likes this.
  7. Shmuckatelly

    Shmuckatelly Newbie

    I'm building an AR308 and am getting a custom barrel made from Lothar Walther. 14.5" light-weight carbine length gas port. But I don't know what size I need to have the gas port drilled to. The gas block I intend to use is a Next Intent adjustable with a full open port of .1406. Should I stick with the industry window of .076-.078?
  8. Jackrabbit

    Jackrabbit Member

    Keep in mind that a .308 gas port will need to be a different size than a 5.56 port.
  9. Jackrabbit

    Jackrabbit Member

    When I've heard that DD barrels are overgassed, it's usually been in reference to the Mk18 upper.
  10. Shmuckatelly

    Shmuckatelly Newbie

    Brain fart. Thats what I get for trying to make a sensible post after a 13 hour shift... Thank you
  11. tcarp1911

    tcarp1911 Newbie

    OP-IMHO,this is excellent info. I have two rifles that are virtually identical, except one has a Sionics barrel and one has a Centurion Arms barrel. The Sionics barreled one runs noticeably smoother, and I'd been wondering what size the gas port is.
  12. Unobtanium

    Unobtanium Member

    Daniel 14.5 middy is 0.076.
  13. Iraqgunz

    Iraqgunz Semper Paratus Arms

    .075-.-77 for the SIONICS
    mtdawg169 likes this.
  14. When teaching Advanced Armorer Courses we measure lots of gas ports, and scope things for erosion. We gauge the gas ports on whatever barrels people bring to Armorer class, which allows us to keep a data base. The couple of Centurion 16" mid-lengths we have come across have measured at .079

    Greg Sullivan "Sully"
    SLR15 Rifles
    (763) 712-0123
    mtdawg169 likes this.
  15. mtdawg169

    mtdawg169 Newbie

    Can this chart be shared outside of the page?
  16. mtdawg169

    mtdawg169 Newbie

    I also have a couple of data points to add. All measured with pin guages in the last six months.

    Noveske N4 12.5" with a 0.071 port. Smaller than I expected.

    BCM 11.5 at 0.073

    PSA 18" rifle gassed - 0.086
  17. Default.mp3

    Default.mp3 Regular Member

    CA 11.5" carbine was 0.073".

    This first run of Hodge 12.5" carbine are 0.068".
    mtdawg169 likes this.
  18. Lobsterclaw207

    Lobsterclaw207 Regular Member

    Can this chart be hosted to Google Forms or something so it can be updated and kept synced?
  19. chrischris

    chrischris Amateur

    I run a Rainier Arms 10.5" Select barrel. It has run flawless with no issues ad my primary duty rifle. I was curious what the gas port size is so I contacted Rainier Arms and was told it's .112".

    This seems on the large size... But it has served me well.
  20. mtdawg169

    mtdawg169 Newbie

    5.56 Chamber with carbine gas system? That is huge.

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