APOD Firearms

AR fail to lock bolt in-place on last spent shell.

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  • SmallBlock850

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    Up until now I have only fed the rifle .223 and never had an issue.

    I was at the range today firing 5.56 \ 55 gr and noticed it failed to lock into open breach the 1st time. (No FtF or FtE)
    Didn't think much of it so I sent another 30 down, same result so I stopped and went to the pistol range.

    The barrel is a 16" (not sure what brand) with 5.56 NATO 1/8 stamped on the business end.
    Lower is an ATI Milsport Multi-Cal.

    Interested to see if there's something I should tweak or replace. Buffer tube spring perhaps?
    1,000+ rounds of .223 no issue.
     

    Cstokes91

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    Were you shooting brass or steel? What was the ejection pattern? Also what buffer weight are you running?

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    SmallBlock850

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    Were you shooting brass or steel? What was the ejection pattern? Also what buffer weight are you running?

    Sent from my Pixel 4 XL using Tapatalk

    Brass - IMI Systems 5.56 \ 55 gr.
    Ejection pattern was standard, most over the right shoulder and maybe 5 hit the fence at 3 o'clock.
     

    Daezee

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    1) I'd clean the rifle, especially the chamber with a brush. Remove and clean the buffer and spring and inside the buffer tube; lightly lube spring and put back into the rifle. Some of the spring lube will transfer to the buffer bearing surface, so no real need to lube the buffer (but you can if you want).

    2) I'd try another magazine...maybe THAT mag combined with THAT ammo don't "play" together well. Is the bolt catch protrusion in that mag follower deformed or chipped?

    3) After 16,000 counted rounds fired, my buffer spring had finally gotten to just under the specified minimum length, so I replaced it. Unless you get a fancy buffer spring, a standard one is only about $5-6.
     

    lil'skeet

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    Sounds like worn mag spring. With 5.56 should be a little more pressure than .223 causing bolt to cycle faster than mag spring can push follower completely up. If new mag/different mag doesn't do it replace 1 steel buffer weight with a tungsten weight.
    I would start with clean firearm if it were me first.
    Heavier buffer weight will also lessen felt recoil. Be careful going too heavy as you will run into cycling problems when you encounter adverse conditions. I like to stay a little over gassed and know I will still cycle at least 3k rounds later without a cleaning.
    It's a balancing act without going to a piston upper or adjustable gas block.
     

    SmallBlock850

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    1) I'd clean the rifle, especially the chamber with a brush. Remove and clean the buffer and spring and inside the buffer tube; lightly lube spring and put back into the rifle. Some of the spring lube will transfer to the buffer bearing surface, so no real need to lube the buffer (but you can if you want).

    2) I'd try another magazine...maybe THAT mag combined with THAT ammo don't "play" together well. Is the bolt catch protrusion in that mag follower deformed or chipped?

    3) After 16,000 counted rounds fired, my buffer spring had finally gotten to just under the specified minimum length, so I replaced it. Unless you get a fancy buffer spring, a standard one is only about $5-6.

    10-4 I'll give it a thorough cleaning. I had already made plans to replace the adjustable stock with a fixed. (tube, stock, and spring) I have no use for the current adj. stock as I always set it at the max rearward setting any how.
     

    Cstokes91

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    10-4 I'll give it a thorough cleaning. I had already made plans to replace the adjustable stock with a fixed. (tube, stock, and spring) I have no use for the current adj. stock as I always set it at the max rearward setting any how.
    I would agree with clean and lube your rifle. ARs like to run wet, this will fix the majority of issues. Try a new magazine as well. Pmag would be the best bet.

    If you're still having the same problem check the buffer weight. If it has an H1 spring, id dial it back to a carbine buffer. I would think if you went heavier it'd make the issue worse. If it has a carbine weight, you can try and H1 buffer but it seems your ejection pattern is already where it needs to be.

    If it is still having issues after this I would think it'd be a problem with the gas system. Either not aligned properly or something else. You could also try another BCG that you know works well.

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    SmallBlock850

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    Sounds like worn mag spring. With 5.56 should be a little more pressure than .223 causing bolt to cycle faster than mag spring can push follower completely up. If new mag/different mag doesn't do it replace 1 steel buffer weight with a tungsten weight.
    I would start with clean firearm if it were me first.
    Heavier buffer weight will also lessen felt recoil. Be careful going too heavy as you will run into cycling problems when you encounter adverse conditions. I like to stay a little over gassed and know I will still cycle at least 3k rounds later without a cleaning.
    It's a balancing act without going to a piston upper or adjustable gas block.

    Happened with 2 identical Magpul mags *I have 3* that are stored empty and very little use. (There's just no way I have worn the springs out that quickly.)
    It has to be something related to the 5.56 ammo or something with the gun.

    I really wanted to confirm it at the range but I didn't bring any .223 with me. :-|
     

    Cstokes91

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    Actually I misread your reply about ejection. When you say over your shoulder would you say that is closer to 5/6 or closer to 430 area.

    3 to 430 is normal, anything over that and it is short stroking (bolt not locking on last round is also a symptom of short stroking).

    If you're running an H1 buffer I'd dial it back to carbine.

    Heres another thread talking about short stroking.


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    SmallBlock850

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    Actually I misread your reply about ejection. When you say over your shoulder would you say that is closer to 5/6 or closer to 430 area.

    3 to 430 is normal, anything over that and it is short stroking (bolt not locking on last round is also a symptom of short stroking).

    If you're running an H1 buffer I'd dial it back to carbine.

    Heres another thread talking about short stroking.


    Sent from my Pixel 4 XL using Tapatalk

    I lube the BCG when I arrive while waiting for the range to go cold.

    I'd be lying to you if I knew for sure as to how the casings landed.
    At the range on Quintette Rd. in Pace each lane is divided by a section of fence.
    95% of my casings land behind the person to my right. 430ish sounds spot on because I always feel weird about sweeping up rounds right behind the person to my right.

    *Now when it comes to short stroking, I originally had an issue (I didn't build this beast) where it wanted to go "full send" (you know what I'm saying) when I 1st got it. I took the rifle to a gunsmith, he replaced a spring at the bottom of trigger assembly mech and machined some sharp spots in the hammer.

    After that, I've had nothing but 100% from it with .223. I did some preliminary research on buffer springs and I keep the device clean.

    I'll start with giving the machine a total tear down cleaning but I have no idea what buffer spring is back there.
    I will have to do some research on whether or not it's possible to decipher that visually.
     
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