Max bbl length for ar308 w/ rifle length gas

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    Duckncover

    Member
    Jan 14, 2021
    77
    howard county
    I built an AR308 with a 22 inch semi bull barrel (I was thinking velocity/accuracy, not weight). Does anyone know how short I can chop this monstrosity to keep it cycling properly with a rifle length gas system?

    EDIT: should have asked for min length bbl w rifle gas system
     
    T

    temccoy

    Active Member
    Nov 13, 2020
    103
    I would suggest you ask the barrel manufacturer the gas port diameter and what length barrel they think will work with that gas port. The good news is if you whack off some of the barrel and it doesn’t cycle, the solution may be to drill the gas port larger, which is feasible. You can’t make it smaller, but you can always make it larger. 20” is the standard length rifle barrel, so with a rifle gas system, it will probably work if you cut it down to that. Hope this helps.
     
    D

    Duckncover

    Member
    Jan 14, 2021
    77
    howard county
    Widening the gas port never occurred to me. I did ask the manufacturer but I figured it might take some time for them to respond so I wanted to see if there were any opinions/experiences here
     
    firemn260

    firemn260

    Active Member
    Sep 15, 2015
    354
    Harford County
    In my limited experience with ar308 barrels, the gas ports seem to be sized for maximum gas volume to begin with.

    It would also seem to me that the larger diameter barrels are drilled with relatively open ports. A friend of mines 20” heavy barrel( I guess you could consider it semi bull) measured .936 at the gas block journal and had a .0930 gas port. It was way over gassed in my opinion.

    My 18” jp barrel IIRC has a .085 port with rifle length gas. I used a slr adjustable block to calm it down.

    I think you may be ok but I’m no expert. I’m just basing my opinions of my own experience with the large AR platform.
     
    erwos

    erwos

    The Hebrew Hammer
    MDS Supporter
    Mar 25, 2009
    13,884
    Rockville, MD
    16" is fine with rifle gas unless your gas port is somehow under-sized. Most people with 18" guns are tweaking the gas down anyways via AGB, there's no shortage of gas in an AR-308 system.
     
    M

    m1carbine

    Member
    Mar 10, 2015
    61
    Like Firemn260 said, 18" is probably a reliable length to go down to. I have a cmmg that is factory configured with a rifle length gassed 18" heavy barrel.
     
    alucard0822

    alucard0822

    For great Justice
    Oct 29, 2007
    17,687
    PA
    The port size and length/dwell will depend to a degree on what ammo you run. 18" rifle gas barrels are popular, most have a port size between .080-.100", can probably do a 16" with around a .100" port to cycle softer nato spec ammo. 308 ammo pressure curves vary wildly, an adjustable gas block is always a good idea, and would allow you to go a little oversized in the port, and tune it back down so you can run a wider range of ammo. Being 308 has so much powder, it tends to favor shorter dwell, many 20+" barrels have rifle+2" gas, and a couple companies make rilfe length gas on 16" barrels, might be able to find specs and copy them.
     
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