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Thread: Reliability problem with AC556F

  1. #1
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    Reliability problem with AC556F

    I have an AC556F that I've had for a couple of years. It was actually my first machine gun. I'd say that it's in very good to excellent condition and it runs 100%. About nine months ago I was fortunate enough to be able to purchase several machine guns from an estate as a package deal. Part of the package deal was another AC556F. It looks identical to my first. However, this one is in like new condition not having been fired for a very long time. When firing the newer one, I immediately noticed that the spent cases do not eject as far as they do from the other. I fired 100rds of Fiocchi 55gr FMJ 223Rem and 90rds of IMI 55gr FMJ 5.56x45 through it. There were multiple instances of the spent cases extracting, but getting caught by the action closing during the ejection process. My guess is that the bolt carrier isn't going back as far as it needs to or it's cycling too fast. I used my trigger pull gauge to estimate the amount of force required to cycle the action back to the point that the carrier encounters a second step of resistance. It took about a pound more force to cycle the new AC556F versus my first one, so that seems to indicate to me that it's not likely that the recoil spring needs to be replaced. Could it be some sort of issue of not getting enough gas? The piston seems to move just fine. I did notice that the four allen head screws on the front sight / gas block assembly don't seem to be screwed in the same amount. The forward ones appear to be screwed in more. I unscrewed them and found that one of the rear ones was very tight, while the others were pretty easy to unscrew. This is odd considering that the one that was tight didn't visually appear to be screwed in as much as the front ones. I don't know if this really means anything though. Is it possible that some gas may have been leaking out? I can soak all the parts and clean everything out.


    Does anyone know what the torque specification is for the allen head screws that trap the barrel between the front sight block and the gas block? I called Ruger and the lady I spoke with was kind enough to leave a message for the armorer, but I have not received a call back. Is there someone who is "the man" when it comes to working on AC556's?

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    Try S&H Arms. Curtis Higgins.

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    Im posting because Ive had similar issues in the past but they go away. Ive had a local armorer who knows them well enough to service them take a look.

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    I'd like to fix it myself if possible. When I attempt to do so, I typically learn a great deal about the firearm in the process. Of course I do need a fallback in case I'm unsuccessful. Thanks for suggesting S&H. I don't really know a local gunsmith that specializes in Mini 14's or AC556's. Plus, I'd rather send it to someone who has seen the problem many times instead of having to learn on the job.

  5. #5
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    Complete detail strip, deep clean, and inspect. You can download a Ruger armorer's manual for the Mini-14, which gives you enough of the AC to be helpful.

    I don't remember the torque spec for the gas block, but remember the rule of thumb being to torque them evenly, as they rarely were.

    Minis/ACs eat firing pins and extractors. Look for burrs on any moving surfaces. My Ruger nomenclature is rusty, but there are several spots that create some drags either with wear or parts interfacing. I'll try to get the book out and review some notes.
    2012 National Zumba Endurance Champion
    الدهون القاع الفتيات لك جعل العالم هزاز جولة الذهاب

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    I previously purchased some factory recoil springs, so I installed a new one in the new (problem child) AC556F when I reassembled it.


    I took both out shooting today. It seems that my memory regarding the spent case ejection distance was incorrect. Both generally seemed to eject the same distance which was about 3 yards at 1:00 on average. However, the new AC556F still had some ejection problems. I started shooting the older once and alternated back and forth. 3x20rds of Fiocchi 55gr FMJ and 20rds of IMI 55gr FMJ M193 went through the older AC556F. Reliability was 100%. 2x20rds of Fiocchi 55gr FMJ went through the new AC556F. There was one failure to eject each time.



    2x20rds of IMI 55gr FMJ M193 went through the new AC556F. It ran 100% through one magazine, but the second had one failure to eject.

    In addition, there were a couple of successful ejections that went left over my shoulder.

    I collected the cases as I shot each rifle and examined them. The cases from the older AC556F didn't show anything unusual. The jammed cases from the newer AC556F all had smear marks on the case heads. Some other case heads fired through it also showed some smear marks while others looked perfectly normal. I'm thinking the smear marks are a clue, but I don't know what to make of them.

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    Try to take some high speed video with an iphone and go frame by frame of a malfunction.

    I dont know much about the AC specifically, but it looks like the extractor is letting go. Maybe try a new extractor and spring (assuming is has a spring).

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    That's a good idea, but I don't know of a way to mount my phone to the rifle. I'm open to suggestions regarding how to do that. I think it has a spring. I may just swap extractors between the rifles to see what happens.

  9. #9
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    Swap complete bolts or extractors.
    2012 National Zumba Endurance Champion
    الدهون القاع الفتيات لك جعل العالم هزاز جولة الذهاب

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    Are the bolts headspaced to the barrel like an AK or are they interchangeable between barrels like an AR15?

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