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bolt over case issue in an ar15

Discussion in 'Rifle Country' started by greyling22, Jan 8, 2019.

  1. greyling22

    greyling22 Member

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    I built an sbr ar15 in 300aac. When I use it with a pmag or a usgi mag I have no issues. When I use my lancer 20 round clear mag I get a bolt over cartridge jam on the 3rd to last round every magazine. If I clear the jam, load the jammed round back into the mag, and either cycle the action or lock the bolt back and release it with the bolt release it all works fine.

    Does that sound like a weak spring to you? or symptoms of some other issue? The mag is only a couple years old and has not been used more than a few dozen times. It worked fine on my blackout using the same ammo before it was a side charging sbr.

    not my picture, but this is what it is doing.
    23kyjjk.jpg
     
  2. someguy2800

    someguy2800 Member

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    If you load one round and fire does it lock back every time? I've had this issue on two different AR's and in both cases the gun was undergassed. On one of them I took the weights out of the buffer and cut an inch off the spring which fixed it, and the other I drilled the gas port in the barrel bigger, which also fixed it.

    Check for leakage around the gas block, gas tube, and gas key.
     
  3. rbernie
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    rbernie Member

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    If the symptom changes with magazines - it's almost certainly a weak spring. I suppose you could be getting bolt drag on the feed lips, but that's far less common. If you do not get the symptom with other magazines, I'd pretty much rule out the gun itself as an issue.
     
    Last edited: Jan 8, 2019
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  4. someguy2800

    someguy2800 Member

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    I missed where you said it works fine with other mags, disregard everything I said, probably just the mag.
     
  5. 243winxb

    243winxb Member

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    To add to post 3. The inside diameter of the mag may be on the tight side. Happens with double stack magazines. The rounds are slowed when moving up, just enough that the bolt missed it.
     
  6. greyling22

    greyling22 Member

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    I'm shooting stumpy 150 grn lead blackout bullets, so drag is a real possibility. But it was working....and the spring isnt' old. But the pmag has a harder to depress spring for sure.
     
  7. Riomouse911

    Riomouse911 Member

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    If the spring came with an unseen flaw making it weak from the get-go, it’s physical age or the numbers of cycles it has gone through won’t really matter.

    Buy a Wolff or other very high quality replacement for it and respring it. If it’s cured, you have licked the problem. If not, it very well could be the mag body itself or the mag follower has some type of flaw internally that’s hanging up that particular round.

    Mags are the easiest (and usually the cheapest) diagnosis and fix for many, if not most, semi auto feeding ills. ;)

    Stay safe!
     
  8. greyling22

    greyling22 Member

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    I just hate to scrap it. Lancers aren't cheap
     
  9. shoobe01

    shoobe01 Member

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    Call Lancer.

    Been a few years, but they have absolutely had out of spec stuff, as happens with mass manufactured goods. They may be able to suggest a fix, send a new simple part, etc.
     
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  10. <*(((><
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    <*(((>< Luke

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    Makes sense it being mag spring related due to the fact that it'll cycle all of them until the last 3 when the spring is at it's least compressed state. And of course because it cycles fine with other magazines.

    Bolt over brass like was being diagnosed by @someguy2800 is usually one of the following issues:

    1. Defective Magazine - springs, bent feed lips, dirty, etc
    2. Undergassed or overgassed
    3. Too heavy of buffer/reciprocating mass
    4. Too heavy recoil spring
    5. Dirty/rough chamber, poor lube in general

    Is the Lancer's metal feed housing at the top damaged in any way, pinched or bent? Seems odd that it failed with as little use as you claim, but like others have said could just be a defective spring.
     
    Last edited: Jan 8, 2019
  11. JWF III

    JWF III Member

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    Add overgassed to that list. It is possible for the bolt to outrun the magazine... Very common in sbrs with suppressors. An H2 buffer fixed my problem.

    Wyman
     
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  12. <*(((><
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    <*(((>< Luke

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    Yes, I’ve heard of that as well, never experienced it. Good point.
     
  13. Slamfire

    Slamfire Member

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    Change the magazine, cut the load. If the reduced load works in the old magazine, the problem was not with the magazine.
     
  14. Riomouse911

    Riomouse911 Member

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    I should have added spring after Wolff, just replace the spring not the whole mag. If that works you’re golden. :)
     
  15. jmorris

    jmorris Member

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    You don’t have to scrap it.

    I have 1911 and AR mags from many different manufacturers, I just use them in firearms that like them and don’t use them in ones that don’t.
     
  16. MachIVshooter

    MachIVshooter Member

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    Where are your cases landing, and does it lock open on empty? Under or over gassed can both cause that condition, the former from short stroking, the latter from excessive bolt speed outrunning the magazine. Since you said it worked fine before SBR, I'm inclined to think you may be on the ragged edge of either condition.
     
  17. greyling22

    greyling22 Member

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    I put an adjustable gas block on it when I made it an sbr. Cranked it all the way down, them opened it up until I got reliable ejection in the 3-5 o'clock range, with most of them happening at 4. Bolt locks back every time on empty. And again, only gives me issues on 3rd from end. If I was mis-gassed you'd think it would happen at different times points.

    I have a call and an email in to lancer. Maybe I can get a new spring out of them. No response yet.
     
  18. cfullgraf

    cfullgraf Member

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    With my 204 Ruger AR-15's, I'd get the bolt over cartridge position jam fairly frequently but not all the time as shown by the OP. I decided the bolt was moving too fast and the magazine spring could not keep up.

    I increased the weight of the rifle stock buffer by scavenging weights out of a carbine buffer but an adjustable gas block should do the same thing.

    This may not be applicable to a 300 Blk, but something to consider.

    Both of my 204 Ruger AR-15's have functioned flawlessly since I increased the weight of the buffer several years ago. On has over 2000 rounds through it without a jam on the prairie dog fields in South Dakota.
     
  19. greyling22

    greyling22 Member

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    Lancer emailed me back and are sending me a new mag. While they didn't attempt to diagnose and solve the problem, you cannot fault that kind of service.
     
  20. MistWolf

    MistWolf Member

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    Of all the problems I've had with severely overgassed shorties, not one was the carrier outrunning the magazine. The usual problem is the extractor jumps the case rim, rendering the ejector ineffective. Usually, the empty gets caught in the action. Sometimes it gets pushed out, but not before it causes a bolt over base malfunction.
     
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  21. d2wing

    d2wing Member

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    I had that problem with a carbine, a heavier buffer slowed the action and solved the problem. A heavier spring did not help. Could be a mag issue though.
    Like JWF, I used an H2 buffer.
     
  22. Jeff Burgess

    Jeff Burgess member

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    I’ll second this comment because this was a issue I had.
     
  23. d2wing

    d2wing Member

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    I also had that problem, besides the H2 buffer, I bought a BCM bolt and extractor. Fixed that. A new extractor spring, donut and extractor may have fixed it too.
     
  24. greyling22

    greyling22 Member

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    Well 3 different magazines have run like tops in the gun, and with the adjustable gas block it shouldn't be (at least in theory) overgassed. I'm going to chalk it up to a bad mag. Or a weak spring. Whatever, it's working now and lancer is sending a replacement mag.
     
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  25. wally

    wally Member

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    I've seen similar on double stack 9mm mags for my AR, although usually its the third round from the top that fails to feed. Usually clears up with some use -- when I can see the finish inside the mag is gone towards the back, then it usually stops having the problem.
     
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