AR15 - How much barrel past the gas block?

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    K

    KRC

    Active Member
    Sep 30, 2018
    616
    Cecil County MD
    Build question:
    On an AR-15 platform.
    Assume rifle length system, mid- load range .223 loads, with an adjustable gas block and adjustable recoil buffer weights/springs.

    What is the minimum length of barrel beyond the gas block that would typically be required to assure proper function?

    In other words, how short a barrel can an AR15 have with a rifle length system and still function reliably?

    I recognize there are many variables, but looking for a general "rule of thumb" minimum value.


    Thanks.
     
    K

    KRC

    Active Member
    Sep 30, 2018
    616
    Cecil County MD
    mdoffroad

    Thanks for the response. The application will not involve a dissipator. (I'd never heard of these, and just read about them.)
    It is, and will stay, a rifle length gas system.

    Again, how short a barrel length beyond the gas port without modifications other than gas port and recoil system tuning?
    Sounds like 4" (for a 16" barrel) would not be reliable.
     
    alucard0822

    alucard0822

    For great Justice
    Oct 29, 2007
    17,706
    PA
    There is a balance between dwell time(length from port to muzzle) and gas pressure/volume(port size/gas block setting/port length from chamber/ammo spec). There is also reciprocating mass and spring rate to consider, generally a 20" M16/AR rifle with 11.5oz BCG, 5oz buffer, 13" port location(.0935" port) and 7" dwell is the gold standard of reliability. The further away from that, the more you have things like increased recoil, increased bolt speed, increased residual pressure in the case(tension in the chamber), or sensitivith to ammo and fouling variables. Of course this is given "normal" gas leakage, alignment of critical parts, and proper assembly/lubrication. A 16" M4 with mid gas gives a similar 7" dwell, but at only 9" port location, pressure/volume is higher, and even H1-H2 buffers at 2.8-3.5oz are lighter than a 5oz rifle buffer, so port size is tuned to allow proper bolt speed, but there is a bit less room for variables, and a defensive build will err a bit more on the overgassed to ensure reliability. Get into more specialized rifles, running consistent ammo, and you can make anything from 16" rifle gas systems work, or really light sub 8oz reciprocating mass setups perform relatively reliably, but in a narrower window. So long story short, it will work, there are tradeoffs, those tradeoffs may not be worth it, there area ton of variables in build and components that may make it better or worse than "average".
     
    clandestine

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,032
    Elkton, MD
    A 15.5" barrel will run with rifle gas and a H2 with a .110 gas port. This is assuming the gas system is efficient and there isn't some crap finish on the BCG (I'm looking at you Nitride and a NiB), and there isn't some gimmicky adjustable bolt carrier, gas key, or wonky buffer system.

    A semi auto AR can run with as little as 2" of dwell time in warmer climates.

    Now if we're discussing an M16, the scenario changes, especially in cold temps and high altitudes.
     
    K

    KRC

    Active Member
    Sep 30, 2018
    616
    Cecil County MD
    Thanks for the informative answers.

    Based on the info provided, I would likely go no less than about 17", so this seems possible given the rifle has an adjustable gas block and a JP Silent recoil buffer which can be tuned with both weights and springs. Barrel is 416R stainless, with no coatings on barrel or BCG.
    This is not a defensive or service firearm, just a range rifle, so perfect reliability, while desired, is not absolutely critical.
    Ammo is hand loaded with premium components, and very consistent.
     
    clandestine

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,032
    Elkton, MD
    KRC said:
    Thanks for the informative answers.

    Based on the info provided, I would likely go no less than about 17", so this seems possible given the rifle has an adjustable gas block and a JP Silent recoil buffer which can be tuned with both weights and springs. Barrel is 416R stainless, with no coatings on barrel or BCG.
    This is not a defensive or service firearm, just a range rifle, so perfect reliability, while desired, is not absolutely critical.
    Ammo is hand loaded with premium components, and very consistent.
    Click to expand...

    Generally the SCS is a problem source. Your going to have problems with it and a short dwell rifle. Abandon that thing and use a Vltor A5 with H0 Buffer if you want something effective.

    I see AR with problems often with the SCS. It's a gimmick.

    If you use an aluminum gas block you will have problems too.
     
    erwos

    erwos

    The Hebrew Hammer
    MDS Supporter
    Mar 25, 2009
    13,886
    Rockville, MD
    Ugh, this thread is making me remember that I need to get an A5 system for my class AR.

    OP, Chad is giving you very good advice. Just get an 18" barrel with rifle gas or a 16" with mid gas, a (new) SLR gas block, and an A5 buffer system. You should not have any cycling problems, assuming you didn't screw up the rest of the build somehow. Buffer weight tuning is on its way out now that adjustable gas is so common; I hate doing the (mandatory) buffer weight game in my 9mm AR SBRs, and if you can avoid it, you should.
     
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