Diagnosing Short-Stroking

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  • jaredm1

    Ultimate Member
    Nov 22, 2008
    1,935
    Shrewsbury
    • Upper is 20" A4 (Bushmaster 1:7 govt. profile barrel assembled by PK Firearms)
    • Lower is built from Del-Ton stripped receiver and RRA LPK
    • Started with rifle-length buffer tube/spring/weight, then switched to carbine/entry length Sully Stock (behavior is the same with either set-up, so I don't think that's the problem)
    • Mix of ammunition: Federal, Centurion, Fiocci, PMC, Wolf, Brown Bear
    • Magazines include P-mags (20 and 30 rnd), and USGI (20 and 30 rnd)

    In general, I will be set up at the range and the rifle will function 100% for the first 100-150 rounds, then begin short-stroking.
    I usually save the steel-cased ammo for last (Wolf, Brown Bear), so the first few range trips, I thought that may have been a factor.
    Just spraying in more CLP will usually help for a while, but seems more like a band-aid than a fix for the root cause. (side note: CLP seems to burn off pretty quickly for me, though I don't think it should burn off this quickly...I plan to try a synthetic motor oil next time to see if there is any change).
    Last range trip, I fired approximately 60 rounds 100%, then began to short-stroke (bolt closing on empty chamber each time, did not hold open at the end of a mag).

    Gas key seems solid, and is well-staked. Buffer tube looks clear; no grime or obstructions. Standard weight carbine buffer and spring are being used. Gas rings seem good.

    So that leaves the gas tube and FSB, correct? I haven't yet assembled an upper, so I'm not sure what to look for here. I know the gas port in the barrel could be out of spec or misaligned with the FSB, or there could be a leak somewhere, or the gas tube could be out of alignment with the gas key, but I don't know how to check and confirm these things. The fact that it doesn't malfunction until several-dozen rounds in, is also puzzling to me. Any help would be appreciated.

    DSC07444.jpg

    carrier rail wear

    DSC07452.jpg

    hammer wear

    DSC07447.jpg


    DSC07454.jpg
     

    SCARCQB

    Get Opp my rawn, Plick!
    Jun 25, 2008
    13,614
    Undisclosed location
    It could be a misaligned FSB that is partially covering the gas port. Heat and debris can cause a partial blockage and result in a short stroke.( Heat causes expansion and will close the orifice a bit.)

    You also need to keep in mind that mixing steel cased and brass cased ammo in AR is bad practice. If you insist on doing so, Fire Brass cased ammo first, followed by steel cased ammo.

    The reason for this is simple, Brass cased ammo will not gum up the chamber like polymer or lacquer that are used to coat steel cases. Residue from firing steel cased ammo will tighten up the chamber. Firing brass cased ammo after that will cause many malfunctions as brass will expand and get cauterized into the chamber.

    You also have to make sure that the gas tube is solidly secured on the FSB. If it rolls during firing, it will decrease the amount of gas going back into the action.

    I hope this helps and good luck with your AR.
     

    gunrunner

    New MK12 Mod 0 Fan
    Dec 20, 2008
    745
    Clear Spring, MD.
    I would guess that your buffer is a standard weight. I have heard of instances of the lower powered ammo like wolf causing buffer bounce. Which slows the buffers momentum and resulting in a short stroke. A heavy weight buffer with its dense tungsten weights will resist the bounce and sometimes fix the issue. Just keep in mind that the wolf and other steel cased ammo is loaded down and is very dirty. Both of which is bad for the reliability of any weapons system.
     

    jaredm1

    Ultimate Member
    Nov 22, 2008
    1,935
    Shrewsbury
    Fire Brass cased ammo first, followed by steel cased ammo.

    This is how I've been doing it.

    And previously, it would only malfunction once I switched to the steel case...so I figured that contributed to the problem (but I would like to get to the point where the rifle will fire anything reliably).

    This last range session however, I fired 60 rounds of brass-cased, switched to Wolf because I had a lot of it, and it short-stroked on the second round. So I switched back to the brass-cased, but it continued to malfunction on nearly every round (it has never been this bad before)
     

    SCARCQB

    Get Opp my rawn, Plick!
    Jun 25, 2008
    13,614
    Undisclosed location
    I forgot... did you check the gas rings.. make sure that they are intact and the gaps are staggered.

    and once you go Wolf, it is hard to go back to brass without a thorough chamber cleaning.
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    That Gas Tube is not aligned correctly, the wear is not even. You can tell if a Gas tube is alligned correctly by inserting a BCG without the Bolt Head. by slowly tilting the gun muzzle dwon the BCG Gas Key should slide over the Gas Key with its own weight. If it binds it not straight.

    -Its possible the Barrel Nut or is torqued incorrectly or is loose causing play, the Gas Tube should be replaced. As SCAR said the FSB could be crooked casuing Gas Tube misalignment that gets worse as the gun get fouled and hot.

    -Run the gun with 30W Syn motor oil

    -Replace the Hammer with rounded Hammer (Get rid of the notched one, it slows and binds the BCG up)

    -That Gas Key is not staked well at all. I would get a new BCM Gas Key, Install it with Red Loctite, torque the New Screws to Spec and Restake.
     

    Russ D

    Ultimate Member
    Nov 10, 2008
    12,029
    Sykesville
    That Gas Tube is not aligned correctly, the wear is not even. You can tell if a Gas tube is alligned correctly by inserting a BCG without the Bolt Head. by slowly tilting the gun muzzle dwon the BCG Gas Key should slide over the Gas Key with its own weight. If it binds it not straight.

    -Its possible the Barrel Nut or is torqued incorrectly or is loose causing play, the Gas Tube should be replaced. As SCAR said the FSB could be crooked casuing Gas Tube misalignment that gets worse as the gun get fouled and hot.

    -Run the gun with 30W Syn motor oil

    -Replace the Hammer with rounded Hammer (Get rid of the notched one, it slows and binds the BCG up)

    -That Gas Key is not staked well at all. I would get a new BCM Gas Key, Install it with Red Loctite, torque the New Screws to Spec and Restake.

    I hope you guys appreciate how valuable it is having this kind of help at our finger tips. :party29:
     
    That Gas Tube is not aligned correctly, the wear is not even. You can tell if a Gas tube is alligned correctly by inserting a BCG without the Bolt Head. by slowly tilting the gun muzzle dwon the BCG Gas Key should slide over the Gas Key with its own weight. If it binds it not straight.

    -Its possible the Barrel Nut or is torqued incorrectly or is loose causing play, the Gas Tube should be replaced. As SCAR said the FSB could be crooked casuing Gas Tube misalignment that gets worse as the gun get fouled and hot.

    -Run the gun with 30W Syn motor oil

    -Replace the Hammer with rounded Hammer (Get rid of the notched one, it slows and binds the BCG up)

    -That Gas Key is not staked well at all. I would get a new BCM Gas Key, Install it with Red Loctite, torque the New Screws to Spec and Restake.



    Sure looks like the key is dragging on the gas tube. I'm not thinking its barrel nut because the wear looks to be on the bottom. But I could be wrong. Tube itself could be a little bent.

    It might be easier to buy another complete bcg from bcm (this way you'll have a spare). This way you can find someone to properly stake your spare.

    As Chad says - lose the hammer.

    Use Slip 2000 EWL for lube and run the gun wet.
     

    Ab_Normal

    Ab_member
    Feb 2, 2010
    8,613
    Carroll County
    That Gas Tube is not aligned correctly, the wear is not even. You can tell if a Gas tube is alligned correctly by inserting a BCG without the Bolt Head. by slowly tilting the gun muzzle dwon the BCG Gas Key should slide over the Gas Key with its own weight. If it binds it not straight.

    -Its possible the Barrel Nut or is torqued incorrectly or is loose causing play, the Gas Tube should be replaced. As SCAR said the FSB could be crooked casuing Gas Tube misalignment that gets worse as the gun get fouled and hot.

    -Run the gun with 30W Syn motor oil

    -Replace the Hammer with rounded Hammer (Get rid of the notched one, it slows and binds the BCG up)

    -That Gas Key is not staked well at all. I would get a new BCM Gas Key, Install it with Red Loctite, torque the New Screws to Spec and Restake.

    I first noticed the gas tube wear and staking job as well.

    Have you ever tried to re-profile (grind) a notched hammer to a rounded hammer?
     

    jaredm1

    Ultimate Member
    Nov 22, 2008
    1,935
    Shrewsbury
    OK, I think I will start with ordering a new gas tube, new hammer, new complete BCG (it won't hurt to buy something quality and then have a spare).

    Chad, I understand what you're saying on how to check the gas tube misalignment with the bolt out of the carrier, but what is the process for finding the cause of the misalignment (it may be obvious, I don't have it in front of me so I'll have to check things out tonight)? I doubt my FSB is perfectly centered because the rear sight windage needed to be adjusted about 5-6 clicks to the right (off of center), but I didn't think that was too unreasonable and there is a bit of tolerance for hole alignment, correct?



    I hope you guys appreciate how valuable it is having this kind of help at our finger tips. :party29:

    Seriously, you guys are great :thumbsup:
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    OK, I think I will start with ordering a new gas tube, new hammer, new complete BCG (it won't hurt to buy something quality and then have a spare).

    Chad, I understand what you're saying on how to check the gas tube misalignment with the bolt out of the carrier, but what is the process for finding the cause of the misalignment (it may be obvious, I don't have it in front of me so I'll have to check things out tonight)? I doubt my FSB is perfectly centered because the rear sight windage needed to be adjusted about 5-6 clicks to the right (off of center), but I didn't think that was too unreasonable and there is a bit of tolerance for hole alignment, correct?


    Seriously, you guys are great :thumbsup:


    Ill be glad to explain things further but Im driving at the moment. Ill be able to talk more later tonight. :)

    If the Gas Bloick is misaligned then a Low Profile Gas Block or Clamp on FSB will be neded to really correct the misalignment. YHM is a good buy either way.

    Your plan is good, order what you can from BCM.

    If your looking for a deal, my buddy at work was going to build an AR and stopped half way and sold everything. He has a brand new BCM M16 BCG that he is selling for $100.00. If you want it let me know. Its never been installed on a rifle.
     
    Ill be glad to explain things further but Im driving at the moment. Ill be able to talk more later tonight. :)

    If the Gas Bloick is misaligned then a Low Profile Gas Block or Clamp on FSB will be neded to really correct the misalignment. YHM is a good buy either way.

    Your plan is good, order what you can from BCM.

    If your looking for a deal, my buddy at work was going to build an AR and stopped half way and sold everything. He has a brand new BCM M16 BCG that he is selling for $100.00. If you want it let me know. Its never been installed on a rifle.

    I'll take the BCG if he doesn't. Let me know.
     

    jaredm1

    Ultimate Member
    Nov 22, 2008
    1,935
    Shrewsbury
    Ill be glad to explain things further but Im driving at the moment. Ill be able to talk more later tonight. :)

    If the Gas Bloick is misaligned then a Low Profile Gas Block or Clamp on FSB will be neded to really correct the misalignment. YHM is a good buy either way.

    Your plan is good, order what you can from BCM.

    If your looking for a deal, my buddy at work was going to build an AR and stopped half way and sold everything. He has a brand new BCM M16 BCG that he is selling for $100.00. If you want it let me know. Its never been installed on a rifle.

    Yes, I'll take the BCG if it's still available (I'm assuming bolt is included?), good deal :thumbsup:

    Rifle-length gas tubes are out of stock at BCM and it looks like they aren't selling the rounded hammers separately.

    Spikes has mil-spec gas tubes in stock, and I hear good things from them, but are these the kind of things you can buy from brownells and not worry about the re-seller? Do you stock any of these parts at Scott's?
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    Yes, I'll take the BCG if it's still available (I'm assuming bolt is included?), good deal :thumbsup:

    Rifle-length gas tubes are out of stock at BCM and it looks like they aren't selling the rounded hammers separately.

    Spikes has mil-spec gas tubes in stock, and I hear good things from them, but are these the kind of things you can buy from brownells and not worry about the re-seller? Do you stock any of these parts at Scott's?

    Yes, complete BCG, everythig is there. Ill let him know you want it. If that dont work Canuk is next in line..

    I have a rifle length gas tube and Rounded hammer at the shop, otherwise Brownells will have them. Get the DPMS Hammer, and DPMS or Colt Gas tube (The Colt will be EXPENSIVE) not sure how far away you are but Ill be there saturday. I can take a look at the rig too in about 10 mins let you know if the gas block will need replacing....no charge to look at it at the counter.


    Let me know my friend.
     

    jaredm1

    Ultimate Member
    Nov 22, 2008
    1,935
    Shrewsbury
    That sounds great, I will take you up on that and buy the parts from you.

    What's a good time to come by on Saturday?
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    That sounds great, I will take you up on that and buy the parts from you.

    What's a good time to come by on Saturday?

    Sounds great, we can move some parts...:D

    Ill check on friday to be sure the parts are there before you make the trip, sometimes they sell my parts and Im not aware if them being gone.

    Ill be in the shop from 11 to 5 on saturday

    Ill likely be in all day friday too but Im not sure yet...
     

    jaredm1

    Ultimate Member
    Nov 22, 2008
    1,935
    Shrewsbury
    Guys, thank you all for your ideas and help.

    Chad, thanks again for hooking me up with the parts I needed and taking the time to explain and do the install. It was a pleasure to meet you :thumbsup:

    I got a chance to take it to the range this morning and had zero problems firing approximately 100 rounds of brass followed by 130 rounds of steel.

    Poor cell-phone pic below:

    IMG00122-20100626-1020.jpg
     

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