Shortstroking?

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  • KIBarrister

    Opinionated Libertarian
    MDS Supporter
    Apr 10, 2013
    3,923
    Kent Island/Centreville
    New 14.5 p&w build
    BA barrel (carbine length)
    .750 low pro Odin works block
    Spikes stainless tube
    Bootleg BCG (single shot straight pull when adjusted to suppressed mode; otherwise as described below)
    VLTOR MUR upper
    Magpul UBR stock
    A5 H2 buffer
    Geissele super 42 rifle length spring

    Port is properly sized (and aligned with bore scope), key is staked, assembled per spec.

    Ejects fine but just barely fails to come back enough to pick up another round. I was taking a new shooter out so I forgot to take a look at the brass. I’ve heard of rough chambers doing this, but haven’t seen that before. This is my first foray with A5. What else might I be missing?
     

    Melnic

    Ultimate Member
    MDS Supporter
    Dec 27, 2012
    15,278
    HoCo
    IF its not picking up the next round then you have to go even further back so that it will hold back on the last round.
    14.5" barrel means the bullet exits 1.5" sooner than a 16" and less gas.
    Lots of possibilities including
    too heavy a buffer
    too heavy a spring
    Bad mag
    Resistance in the BCG
    Rough chamber
    Light ammo
    and more.

    Some of these are less probable than the other.
    IF you have another AR, you can start by swaping things one at a time.

    For a new AR, I load 1 round at a time 10 times to make sure I get the last round hold back.
    If I"m diligent, I also do this in the winter when its near or below freezing temps.

    Most of my ARs have 3oz buffer weights including my 14.5" upper w/ a pinned/welded flash hider that was surplus from Iraq security contract. The upper and BCG was very broken in.
    I have a 300BLK upper that uses a heavier buffer though.

    Others will chime in with more and likely better advice.
     

    F-Stop

    Ultimate Member
    Feb 16, 2009
    2,484
    Cecil County
    Just looking at your list quickly I’d swap bcg and test. Something about that venting bcg makes me think it could be bleeding gas.


    Sent from my iPhone using Tapatalk
     

    camo556

    Ultimate Member
    Aug 29, 2021
    2,634
    why the H2 buffer and rifle length spring for a carbine 14.5" gas?

    my guess it's its undegassed for the spring and buffer.

    try a springo white standard power and/or an h1 buffer.
     

    camo556

    Ultimate Member
    Aug 29, 2021
    2,634
    not familiar with the bootleg BCG, but I googled it and seems like shortstroking is not uncommon. I see comments like "bleeds a **** ton of gas". the fix is to play around with lighter buffers and springs until it's tuned.
     

    DavidA

    The Master of Disaster
    Dec 6, 2013
    371
    Annapolis
    Swap out BCG if you can readily get one or barrow one from some one close to you and establish if the BCG is the problem. If not check chamber and possible go to a lighter buffer.


    Sent from my iPad using Tapatalk
     

    SkiPatrolDude

    Ultimate Member
    MDS Supporter
    Oct 24, 2017
    3,331
    Timonium-Lutherville
    why the H2 buffer and rifle length spring for a carbine 14.5" gas?

    my guess it's its undegassed for the spring and buffer.

    try a springo white standard power and/or an h1 buffer.

    A5H2 is the standard buffer for the A5 set up and utilizes a rifle length spring.

    Sprinco white is for carbine length tubes.

    Sprinco green would be appropriate for here, but I’d change buffer weight before I changed the spring rate.

    OP - if you think the Geissele spring may be a factor you could buy a ~$5 rifle length spring for testing. I don’t have personal experience with the Geissele spring, so I can’t speak for its performance.

    Your issues aside, seems like a nice build once you work the kinks out.
     

    camo556

    Ultimate Member
    Aug 29, 2021
    2,634
    A5H2 is the base buffer for the A5 set up.

    Sprinco white is for carbine length.

    Geissle claims the super 42 is 15% stronger than standard.

    lots of comments about this bcg shortstroking and needing to be tuned with lighter springs/buffers.

    some combination of a lighter buffer and spring ought to fix it.
     

    KIBarrister

    Opinionated Libertarian
    MDS Supporter
    Apr 10, 2013
    3,923
    Kent Island/Centreville
    I just realized I had a spare BCG in my ammo crate. Too focused on keeping an eye on my new guy to even think (insert Homer Simpson “Doh!”). Thanks fellas, I’ll swap in a known mil spec BCG next range trip. It’ll probably be a few weeks, but I’ll report back when I give it a shot.

    That will be depressing if it’s the bootleg BCG. But I should no better!
     

    SkiPatrolDude

    Ultimate Member
    MDS Supporter
    Oct 24, 2017
    3,331
    Timonium-Lutherville
    Geissle claims the super 42 is 15% stronger than standard.

    lots of comments about this bcg shortstroking and needing to be tuned with lighter springs/buffers.

    some combination of a lighter buffer and spring ought to fix it.

    There’s no good reason it shouldn’t run well with the set up he’s got, so I’d strongly suspect some gas inefficiency. BCG is the first place to check since the OP mentioned the gas port is correct size.

    Reasons like this are why I don’t use adjustable anything.
     

    KIBarrister

    Opinionated Libertarian
    MDS Supporter
    Apr 10, 2013
    3,923
    Kent Island/Centreville
    Geissle claims the super 42 is 15% stronger than standard.

    lots of comments about this bcg shortstroking and needing to be tuned with lighter springs/buffers.

    some combination of a lighter buffer and spring ought to fix it.

    I have a stack of mil spec buffer springs. I’ll try it both ways. But I’d rather go milspec with the BCG than start having springs compensating for it
     

    Rockzilla

    Ultimate Member
    Feb 6, 2010
    4,516
    55.751244 / 37.618423
    port chart.jpg

    A Snippet......
    Ejecting at 4-5 o'clock at 20 degrees is equal to 2-3 o'clock at 90 degrees, IME. Cold weather will slow
    down the cycle. If a rifle is barely cycling in hot temperatures it will short stroke when it's freezing outside.
    Short stroking is usually caused by too much friction on the BCG or too little gas. Below are some thing to
    check and possible areas that could be improved upon to get the carrier to travel with more force/speed.
    Since you are barely short stroking it may only take a small adjustment here or there to relieve friction?
    I'm not familiar with your buffer spring but swapping it out for a standard carbine spring would be the first
    thing I would experiment with. Maybe your spring system doesn't like cold weather? If you want to stick
    with the same spring you can drop the buffer weight down or find something in the list below that can be
    improved upon.

    Possible reasons for too little gas=
    Too small of a gas port.
    Gas leaking at the gas block/barrel connection.
    Gas leaking at the gas block/gas tube connection.
    Loose gas key.
    Gas block not aligned with the gas port.

    Possible reasons of added friction=
    Rough/tight chamber - stuck cases.
    Buffer or buffer spring too heavy.
    bolt/barrel extension not timed correctly.
    Gas key/gas tube not aligned correctly.
    Overly tight gas rings.
    Receiver extension not aligned with the receiver.
    Receiver misshapen.
    Rough surface inside receiver or extension.
    Foreign material loose in the carriers path.
    Friction from the mag or the next round in the mag on the carrier.

    ---Unable to find my actual PDF..

    -Rock
     

    Boats

    Beer, Bikes n Boomsticks
    Mar 13, 2012
    4,073
    Howeird County
    The UBR uses a CARBINE buffer, not an A5 out of the box

    (did you forget to install or remove the buffer spacer for A5 buffer?)
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    I see issues like this often with anything adjustable. Adjustable bolt carrier tend to be very problematic.

    I wouldn't pay attention to the ejection pattern. It means nothing if brass pattern is somewhat consistent and it ejects with authority.

    The A5 is a great system but not for all setups. I definitely wouldn't run a geissele rifle spring with an A5 unless it was really gassy.

    Keep the brass next time you have problems. It can help diagnose issues.
     

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