Gas port size in chopped Patrol | SIG Sauer 556 Forum

Gas port size in chopped Patrol

Discussion in 'Technical Discussions & Troubleshooting' started by CGRBB, May 28, 2020.

  1. CGRBB

    CGRBB Active Member

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    I have a 556 Patrol that I had cut to 12.5”, in an attempt to achieve a 553LB vibe. Proportions are excellent, but damn near anything is better than the mile of bare barrel on a factory Patrol!

    Upon reassembly and initial firing though, I found it was getting way too much gas. I then went through all my valves, thinking I had mixed them up, but none are any different. Either I got an oddball, or Sig didn’t bother using a different valve in the Patrol model. The gun had been apart so long, I honestly don’t remember how it shot before.

    Long story short, I had the same gunsmith that cut the barrel modify a valve so that it has detents on the opposite tang as well, enabling me to drill two additional ports; thereby creating a 4-Pos valve.
     
  2. CGRBB

    CGRBB Active Member

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    It’s no walk in the park to drill through these valves. Got to get through the chrome first, then it’s still tough for the bit to bite it. After the gunsmith worked some magic on the bit, it drilled through. Ended up picking a .042 bit to start. It ended up being about perfect. Ejects just like my other Sigs, and doesn’t kick like a mule anymore.

    1.0668mm is the conversion. So really only a hair bigger than the smallest port on a SAN 4-POS valve, which is advertised as 0.9mm, and is intended for silencer use.
     
  3. CGRBB

    CGRBB Active Member

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    I remembered two things just now, that are relevant to my previous posts. This valve I modified had been previously modified by me for some reason I can’t recall. I enlarged one of the existing gas ports. It could be that I enlarged the one that was originally the correct size for regular operation in the Patrol (oops).

    Also, it doesn’t make sense to compare short gas system porting in a US 556 to a 553LB/SB, because the SAN piston is a smaller diameter and shorter length.
     
  4. Reformer

    Reformer Member

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    Thanks for all the information. I am looking at doing something similar.
     
  5. muffe

    muffe New Member

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    Wouldn't you be getting less gas with a shorter barrel length, as the dwell time is decreased?
     
  6. CGRBB

    CGRBB Active Member

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    Yes. That’s why I think I misplaced the original Patrol valve, or Sig Sauer just did a terrible job determining proper port size, or just didn’t care.
     
  7. Nacomah

    Nacomah New Member

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    Quick question, I have a sig p556 With a 10” barrel that I have being changer out for a 16” if it is capable of being done . Will I need to do any thing to the valves or any other mods ?
     
  8. CGRBB

    CGRBB Active Member

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    That is an ill-advised project, in my opinion. You will almost certainly run into gas port timing and headspace issues. Not to mention potentially large gunsmith fees. There is also a legal grey area too, considering your receiver has a pistol designation. If you really want a 16” gun, I’d recommend buying a 556 or 551A1.
     
  9. Nacomah

    Nacomah New Member

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    Every one keeps saying that , sounds like it’s the best idea. Thanks for the advice it’s well received.
     
  10. fachries

    fachries Member

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    +1
    [​IMG]
     
    muffe likes this.

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