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ar15 18" barrel rifle length gas system?

Discussion in 'Rifle Country' started by D51208, Feb 1, 2012.

  1. D51208

    D51208 Well-Known Member

    Hi all,

    I am looking to buy an Ar-15 18" barrel with a rifle length gas system. I don't want to pay a huge chunk of cash, nothing more than $250. I have searched quite a bit, but have only found a few. Dpms was one of them. Stags 3GH upper was another, but i cant toss $775 on an upper.:banghead:

    Also thoughts on just getting a 20" barrel with the specification i need and sending it to get cut, re-crowned and threaded? Know of any places?
  2. GrandmasterB

    GrandmasterB Well-Known Member

  3. Tirod

    Tirod Well-Known Member

    Why 18"? It's a net loss with 5.56, and other intermediates work better longer or shorter.

    There's a reason so few 18" rifle gas barrels are out there - the ballistic performance compromises just about any popular cartridge used in the AR15.

    Time to back up a review what the rifle is being tasked to do, and whether the ballistics match up. What's the range and target, does the cartridge match, what's the optimum barrel length for that?

    Match the optimum cartridge to the application, and the optimum length for the barrel. With that settled, then the gun will perform at it's best, rather than a compromise.
  4. HJ857

    HJ857 Well-Known Member

  5. MtnCreek

    MtnCreek Well-Known Member

    +1. What's the target and what's the range?

    If you're building an SPR type, then the WOA barrel mentioned above is great.
  6. chrome_austex

    chrome_austex Well-Known Member

    whats wrong with an 18" barrel? I mean people use 16" barrels all the time w/o people giving them grief.
  7. D51208

    D51208 Well-Known Member

    Grandmaster, thank you for the information. I found it extremely helpful!

    TiRod: I was under the influence that nothing was compromised, and that ballisticly there is only a 75ftps difference between the 18" and 20" barrel. Care to elaborate which intermediates are better for curiosities sake? I haven't bought a barrel yet, so as long as the ballistics are good and its not over 20" it might be worth looking into. Let me know.

    Chrome: The way i understand it is that the 18" has the maneuverability of the 16" barrel with the ballistics of the 20" barrel.
  8. Jim Watson

    Jim Watson Well-Known Member

    I had one, thought it was very handy.
    But then I got it by cutting a longer barrel so the cost was small for a setup that did everything for me that a 16" would.

    If the tube was salvaged after The Incident, I was thinking about returning it to use.
  9. Waywatcher

    Waywatcher Well-Known Member

    In my research, I found the 18" rifle gas to be less than ideal. If you cut and crown a 20", you will need to bore the gas port out to ensure proper function.

    The dwell time in a gas system leads me to believe the following are ideal:

    14.5"= Carbine gas
    16.0"= Midlength gas
    18.0" = Intermediate gas (See Novekse)
    20.0" = Rifle gas

    When one steps outside of the AR paradigm, one realizes that 20" in the big scheme of things is still pretty short for a rifle. It is also the gold standard of reliability, and has the most velocity. The decision was easy for me for a Rifle.
  10. Zerodefect

    Zerodefect Well-Known Member

    18" is the longest I go in AR's now. 5.56 is the limiting factor for the range of that rifle. If I want to reach out further, then I'm looking at a 20" AR10 or Bolt action rifle.

    My 18" BCM 410SS MK12 mod 3x SPR is a real smooth shooter. It has it's niche in my collection.

    Rifle length gas is the only way I'd get an 18". It's pointless to run cheap underpowered ammo in a precision SS barrel setup for longer ranges. Save that ammo for your chrome carbines.

    If a chrome bore carbine is all you're after, stick with 14.5-16" and 20".
  11. gotigers

    gotigers Well-Known Member

    My 18" rifle is my favorite. I use it for 3 gun. Home built with a DPMS barrel medium contour, 1:8, NATO chamber. It is as accurate as my 20" SRS match AR. It is a criterion HBAR, 1:8 wylde chamber. My 18" functions flawlessly. Never had an issue. I haven't noticed any drop off at 200 & 300 yards vs my 20".

    Most 3 gunners use 18" rifles. At least what i've seen and read about.

    I prefer 18" rifle over any length gas combo.
  12. Tirod

    Tirod Well-Known Member

    Waywatcher brings up one factor - when barrel length is altered for a cartridge, then gas port diameter gets changed to compensate. One scenario is when carbine gas on a 16" gun is set up to shoot low powered ammo - the gas port is opened up by some makers and it's more than needed for military TAC ammo.

    Another point is that 16" really should be mid length gas; the correct determination of gas length isn't how far out from the chamber, but how far from the muzzle. Five to seven inches is where it belongs to set the correct timing.

    Barrel length for an alternate cartridge is part and parcel of the designers total package. Each one is set up with the ratio of case to bullet diameter, and how fast a powder can be tolerated. With the dozens of cartridges available, there is NO one length that is correct for all. One cartridge with little change in case to bore diameter could use much faster powder, and have an optimum barrel length that is short. Change that to a larger case diameter with much smaller bullet diameter, pressure rise has to be controlled, a slower powder used, and a longer barrel is needed to get the most out of it.

    Lots of shooters are using longer barrels to get a few more feet per second, when they could have just developed a hotter load, or moved up to the next grain weight bullet. That tends to make them longer, can improve the BC, and give better terminal performance. Barrel resonance is a factor too, the optimum length is better selected by cutting the barrel at the muzzles least point of vibration.

    There's a lot more ways to get another 100fps, and that isn't always the better choice. One thing is definite fact, tho, a cartridge designer is quite specific about their range and target goals. Picking one based on market spin or popularity isn't necessarily valid for what a specific shooter is trying to do.

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