New Member in need of AR-15 Guru.

Discussion in 'Gunsmithing' started by SBruce, Dec 13, 2009.

  1. SBruce

    SBruce Well-Known Member

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    Oct 31, 2009
    Hello,

    I know this isn't really an AR site, but it's the only one I am registered on. Just recently posted my introduction but since I am here; I've got a situation with a Bushmaster Predator AR-15 .223 that is short cycling more often than not. Bolt wont lock open on an empty magazine most of the time. Seems to be worse when its below 30 degrees. The rifle has a jewell trigger in it, is well broke in and has been cleaned numerous times. It's done this off and on since I bought it. I've tried different magazines and different ammo, I've even tried "specialty lubes".

    If I load more than one round in the mag:

    Sometimes it will chamber the next round.
    Sometimes it will partially chamber the next round and jam.
    Sometimes it will close and lock on an empty chamber and leave the next round in the magazine.

    I've checked everything I have heard of possibly being the culprit.
    Gas port seems to be open and large enough (.085" plastic straw easily goes all the way into and out again).
    Gas tube and block seem to be open and clean (I can easily blow through them and have even flushed them w/ contact cleaner).
    The holes in block and barrel look to be in perfect alignment.
    Carrier key doesn't seem to bind on tube and both appear in to in alignment.
    Carrier key bolts ARE tight and have always been staked.
    Buffer tube, buffer and spring are clean; I can't feel any binding when cycling ammo through the action by hand. Buffer weighs about 6 oz. and spring feels strong.
    Gas rings seem ok, bolt won't fall out of carrier and there is slight resistance to moving the bolt inside the carrier with fingers. I also stagger the gaps when I clean it.
    Problem doesn't seem to be related to ejection or extraction. The ejected cases all land about 3 feet from gun and close to each other. There are only very slight extractor marks on ejected cases (usually not enough to catch a finger nail on). The bolt and barrel lugs seem smooth and I don't feel any binding when engaging or disengaging the bolt with a finger tip.
    I've tried everything I can think of other than sending it off.
    I've thought that maybe the buffer is too heavy, and have heard that lighter ones are available??

    Any help would be greatly appreciated.
    Any ideas of other easy things I can try before sending it off would be appreciated.
    Anyone know of a good reputable AR Guru in the Wyoming, Colorado, Montana or South Dakota area.??

    Thanks in Advance and Happy Holidays.



     
  2. NomadPilot

    NomadPilot Well-Known Member

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    Where does your brass land? 3:00? 1:00? A2 stock with rifle buffer? There are varied weights in buffers, but usually that's more for carbines than rifles.
     
    Last edited: Dec 13, 2009

  3. NomadPilot

    NomadPilot Well-Known Member

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    510
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    Last edited: Dec 13, 2009
  4. NesikaChad

    NesikaChad Well-Known Member

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    Reading your post you sound like you've gone through the bulk of the possible issues.

    A good thing.

    What's the round count on the gun and how is it used? is it a service rifle? I ask cause Jewel triggers are popular with them and S/R shooters typically consume more ammo than most.

    Have you checked the wear on the end of the gas tube? Believe it or not these are consumables and do need to be replaced. The "bulb" on the end that acts like a partial seal will wear over time. It may need to be replaced.

    Next, have you changed ammunition/bullet weight at all? Direct gas operating systems are more sensitive to these changes. If all else fails, perhaps try shooting some 75/80 grain bullets through the thing and see if it duplicates the condition.

    How did you check your gas rings? When I went to the Colt school and the supplemental course that the State Dept. has I was told that the gas rings should support the weight of the bolt carrier with the bolt at full extension. Just pull the bolt and set the whole assy on a table. A firm rap on the table with your fist should allow the bolt to collapse somewhat. If it drops all the way then the rings are probably shot.

    If you go with the one piece spiral rings there is a break in period. In 2001-03 I spent a fair amount of time with the USMC-R shooting team and they experimented with these types of rings quite a bit. They did see an improvement in how the guns held elevation at 600 yards but they were a little more temperamental at first. It takes about a hundred rounds for them to settle in. They also wear faster than the single rings but this may be due to the elevated loads that are typically used when shooting 80 grain bullets at 600 yards.

    Hope this helped.

    Chad
     
  5. SBruce

    SBruce Well-Known Member

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    Thanks for your responses, gives me a couple more ideas.

    to answer your questions:
    Brass lands about 3:30 or 4:00.
    Believe it to be the A2 Stock, it has the trapdoor butt and is pretty standard looking. Yes, I've checked the vent hole and it's clean and open.

    I've probably fired somewhere around 600 rounds through the gun. At least 500, probably more.
    I use it for Varmint Hunting.
    I haven't mic'd the tube bulb.......good idea thank you.
    I have tried heavier bullets but that was earlier in the guns life.
    currently shooting the Black Hills 60 grn loads. I really want them to work, because I've got 600 rds of that left..Plan on reloading for it after I shoot them up.
    I checked the rings w/ the "inversion" method, where you hold the carrier w/ bolt upside down(bolt face down) and gently shake it vertically.......bolt shouldn't fall out, and it doesn't. Partially disassembled BCG of course. I'll try the table top method and if it looks loose I'll change the rings, I've got some extra ones here now.

    Thanks again

     
  6. NesikaChad

    NesikaChad Well-Known Member

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    A new gun with only 600 rounds through it shouldn't be having gas ring or gas tube issues. Not unless your going "Blackwater" on your dog fields. Sustained rate of fire on these is about 12-15 rounds/minute.

    I'd start looking elsewhere. Ensure your cam pin isn't binding at all. This could delay coming out of battery on extraction and lead to a lethargic action/cycle.

    Good luck.

    C
     
  7. partisan1911

    partisan1911 Well-Known Member

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    What brand of magazines are you using? It sounds like you are using GI spec metal magazines. The issues you are having sounds like your metal magazines have a crack which would expand a little when it is cold out. Depending on the amount of ammunition you put in the magazine will expand/retract the crack a little bit. I have had good luck with the HK magazines$$$ but have recently started using the P-mag. I have used the P-mag at -30F and 120F with no issues.
     
  8. SBruce

    SBruce Well-Known Member

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    The carrier did fall when using the "table top" method, not alot but it did move.
    I changed out the gas rings anyway and the bolt feels quite a bit tighter now.

    Ammo seems to cycle more reliably now, but bolt still won't lock back on empty mag unless I pull it back by hand.

    Tube and key clearance mic's out at .005" (key is .180 and tube is .175)
    I ordered another tube from Bushmaster anyway, don't know when it will show up though. Plan to replace it and try again.

    I also ordered a JP Enterprises "low mass buffer" weighs about 1/2 of what my buffer is. I'll try the buffer and maybe even a new gas block from JP if the new tube doesn't do the trick.

    Cam pin seems very free, but if it's binding up inside the receiver that will probably be a gunsmith fix I am assuming? Am also assuming that if it needs another bolt and/or carrier, that will be a smith issue too due to headspace issues?

    Thanks again Chad
     
  9. SBruce

    SBruce Well-Known Member

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    I have been using Colt 20 rnd and all sizes of Bushmasters stuff.

    Haven't ever used any of the polymer stuff, if I can find one I'll give that a shot.
     
  10. NesikaChad

    NesikaChad Well-Known Member

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    You may have an issue with the bolt going into battery with the lighter buffer.

    The weights inside a buffer serve a very specific purpose.

    Ever frame up a wall with a big nasty framing hammer? You whack away and give it that last good one to really seat the nail and what happens? The joint separates and you have hit it again.

    The bolt will attempt to do the same thing. The spring will drive everything home and the bolt will rotate into battery, only to bounce back out and rotate the lugs out of battery.

    The weights inside the buffer keep on whacking and keep the bolt locked up till everything settles down.

    Just be forewarned in case the bolt doesn't want to stay put.

    Chad
     
  11. SBruce

    SBruce Well-Known Member

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    Problem appears to be solved.

    The new gas tube was .004" bigger, changed it out.
    The tube was a little loose in the block, changed the block out.

    Carl at ASI said that he uses a .1 port hole on his customs, so when I went through Denver last week I stopped by his shop and he opened the port to .103".

    Went out last night (about 15 degrees and near zero with wind chill) fired 20 rounds single shot and let the rifle get good and cold again every few rounds. Bolt locked back every time. Has had about 80 rounds fired since cleaning. Last week, with only 60 rounds fired; Bolt wouldn't lock back on empty magazine.

    It appears that the issue was a combination of things; rings wore out, tube ends too small, gas block too big and perhaps too small of a gas port. Whatever the case, all seems to be good now.........Yee HA!......HAPPY NEW YEAR!!