View Full Version : 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.
https://i.postimg.cc/MTVhzp4z/IMG-9153.jpg
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?
Try S&H Arms. Curtis Higgins.
TheChunkNorris
10-16-19, 21:36
I’m posting because I’ve had similar issues in the past but they “go away”. I’ve had a local armorer who knows them well enough to service them take a look.
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.
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.
I previously purchased some factory recoil springs, so I installed a new one in the new (problem child) AC556F when I reassembled it.
https://i.postimg.cc/y8YG6ZvS/IMG-9244.jpg
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.
https://i.postimg.cc/XvNBzVNW/IMG-9234.jpg
https://i.postimg.cc/W49qRQtk/IMG-9235.jpg
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.
https://i.postimg.cc/ZR29np8Y/IMG-9240.jpg
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.
https://i.postimg.cc/W1Cx94wF/Smear-marks-on-case-heads.jpg
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).
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.
Swap complete bolts or extractors.
Are the bolts headspaced to the barrel like an AK or are they interchangeable between barrels like an AR15?
TheChunkNorris
10-21-19, 08:26
Are the bolts headspaced to the barrel like an AK or are they interchangeable between barrels like an AR15?
They are head spaced but I’ve seen people swap them.
Are the bolts headspaced to the barrel like an AK or are they interchangeable between barrels like an AR15?
Officially, bolts are "fitted" and headspaced. Ruger is funny about bolts and bolt components. Practically, you can swap for this diagnosis. Use high-quality domestic .223 ammo.
I have three different 223Rem loads, so that shouldn't be a problem. I also have a headspace comparator, so I can immediately check the spent cases to see if they're within spec. Now that I think about it, I don't recall seeing a serial number on the bolt. If it's critical that the bolt remain with the rifle, you'd think the bolt would be serial numbered. They could be easily mixed up in an armory. Of course I could have missed it.
I have three different 223Rem loads, so that shouldn't be a problem. I also have a headspace comparator, so I can immediately check the spent cases to see if they're within spec. Now that I think about it, I don't recall seeing a serial number on the bolt. If it's critical that the bolt remain with the rifle, you'd think the bolt would be serial numbered. They could be easily mixed up in an armory. Of course I could have missed it.
No serial numbers unless specifically requested on a contract. Ruger's restricts bolts and FPs to factory service only citing required fitting, even for armorers. Last I ordered, extractors were available though.
FPs got to be a pain, and if sending a gun in I would usually ask for a spare bolt or spare FP to be fit while it was there.
I did receive an email reply from Ruger today regarding the screw torque specifications. The gunsmith stated "We set the torque spec here at the factory with air pressure, not a normal torque specification. Screws do not require a great amount of torque. The torque of the screw is not what holds the screw in place but rather staking the threads of the screw is what holds them in place. The low screw tension gives the barrel some room to expand while shooting. This is particularly important in the AC-556." So that's definitely helpful information, but I don't know how anyone can reinstall the screws without knowing the torque specification. I guess the bottom line is that I need to get some more screws and then have someone stake them to "low screw tension".
I did receive an email reply from Ruger today regarding the screw torque specifications. The gunsmith stated "We set the torque spec here at the factory with air pressure, not a normal torque specification. Screws do not require a great amount of torque. The torque of the screw is not what holds the screw in place but rather staking the threads of the screw is what holds them in place. The low screw tension gives the barrel some room to expand while shooting. This is particularly important in the AC-556." So that's definitely helpful information, but I don't know how anyone can reinstall the screws without knowing the torque specification. I guess the bottom line is that I need to get some more screws and then have someone stake them to "low screw tension".
Interestingly, one of the several tweaks for mini accuracy is to torque the screws tighter than spec and evenly.
I sent pictures to Ruger and received another reply from the same helpful gunsmith. He said that based on the pictures he would suspect that the extractor is not holding the case tight against the bolt face so he suggests checking the extractor tension. It may be as simple as needing a new extractor spring. Hopefully I'll get a chance this weekend to field strip both rifles and compare the extractor tensions.
I decided to swap all of the bolt parts (save for the bolt and firing pin) between both rifles. This rifle is already a PIA because you have to remove the pistol grip to remove the trigger group assembly to remove the stock to remove the recoil spring to remove the op rod to remove the bolt. However, once I got down to disassembling the bolts it went from PIA to near impossible. After stabbing myself in the thumb with a screw driver and about two hours of trying different tricks, I finally got both bolts disassembled.
https://i.postimg.cc/WpyRj5B3/IMG-9249.jpg
I could find absolutely nothing that seemed wrong with the bolt parts from the one that's having the problems. So, I swapped all the bolt parts save for the bolt and firing pin. Fortunately, reassembly of the bolts were easier (not easy) in reverse order.
https://i.postimg.cc/YqL5S17x/IMG-9250.jpg
Unfortunately, reinstallation of the trigger group assemblies were both a PIA as well. I can't imagine a military force of much size maintaining these rifles. Now that both rifles are back together with their respective bolt bits swapped, I'll take them shooting tomorrow to see if the reliability issues follow the bolt bits or stay with the same rifle.
I took both of my AC556F's to the range Sunday. I put 50rds of Fiocchi 55gr FMJ through the older one (that has the bolt pieces from the newer one installed) and it ran 100%. I put 100rds of Fiocchi 55gr FMJ and 60rd of IMI 55gr FMJ through the newer one (that has the bolt pieces from the older one installed) and six failures to eject and or feed. There were also four instances when the action hesitated momentarily. Conclusion: The extractor and ejector are not the problem.
I took both of my AC556F's to the range Sunday. I put 50rds of Fiocchi 55gr FMJ through the older one (that has the bolt pieces from the newer one installed) and it ran 100%. I put 100rds of Fiocchi 55gr FMJ and 60rd of IMI 55gr FMJ through the newer one (that has the bolt pieces from the older one installed) and six failures to eject and or feed. There were also four instances when the action hesitated momentarily. Conclusion: The extractor and ejector are not the problem.
Action hesitated how?
While shooting?
While manually cycling?
Bolt traveling rearward or forward (if you could tell)?
At least you can eliminate a few parts.
Action hesitated how?
While shooting?
While manually cycling?
Bolt traveling rearward or forward (if you could tell)?
While shooting. It's hesitating when cycling on full auto. There's no way to know whether it's happening when the action is going forward or rearward.
I swapped the bolt components back to their original rifles and then swapped the stocks and trigger packs. Two weeks ago I shot them again. The older AC556F ran 100% with the stock and trigger pack from the new rifle installed. The newer AC556F had multiple failures to extract fully, spent cases caught when the action when back forward and several hesitations while cycling. Conclusion, the stock and trigger pack are not the issue.
Yesterday, I swapped the stocks and trigger packs back to the original rifles. I then cleaned the chambers and checked the headspace of each rifle using my 5.56x45 headspace gauges. Neither rifle would close on a Go gauge. I then swapped bolts between the rifles. Again, neither would close on a Go gauge. During the process, I used my Hornady headspace comparator tool to take some measurements and found the following:
Manson 5.56x45 Go Gauge: 3.461"
Manson 5.56x45 NoGo Gauge: 3.467"
Fiocchi 223Rem 55gr FMJ cartridge: 3.456" to 3.457"
Spent Fiocchi 223Rem case from old AC556F: 3.457"
Spent Fiocchi 223Rem case from new AC556F: 3.460"
Norma 223Rem 55gr FMJ cartridge: 3.467" to 3.468"
Spent Fiocchi 223Rem case from old AC556F with the new AC556F's bolt: 3.462" (measured after shooting the same day)
Spent Fiocchi 223Rem case from new AC556F with the old AC556F's bolt: 3.459 to 3.460" (measured after shooting the same day)
Based on the fact that neither rifle would close on a Go and the spent case measurements are all less than the Go measurement, it seems to me that both have tight headspace.
Here's what happened when I fired each rifle.
New AC556F with the old AC556F's bolt:
1x30 Fiocchi 55gr FMJ: 1 failure to completely extract the cartridge.
1x20 Fiocchi 55gr FMJ: 100%
1x25 Fiocchi 55gr FMJ: 100%
1x30 IMI M193 55gr FMJ: 100%
1x20 Norma 55gr FMJ: 100%
Old AC556F with the new AC556F's bolt:
1x30 Fiocchi 55gr FMJ: 1 hesitation during cycling.
1x20 Fiocchi 55gr FMJ: 100%
1x25 Fiocchi 55gr FMJ: 1 hesitation during cycling.
1x30 IMI M193 55gr FMJ: 100%
1x20 Norma 55gr FMJ: 1 failure to eject where the spent case got caught by the action going forward.
So this is the first time that I was able to make the old AC556F have any sort of reliability issue. The only thing that was different was the bolt being swapped between the two rifles. I really don't know what to conclude here. It seems that swapping the bolt definitely caused the old reliable AC556F to have issues, so that would lead me to believe that the bolt in the new AC556F is the problem. However, there was that failure to completely extract the cartridge when shooting the first magazine through the new AC556F with the old AC556F's bolt.
TheChunkNorris
11-18-19, 02:48
I know this is going to sound silly but do you have any 62gr ammo?
Yes. However, this rifle was manufactured when 55gr was the thing and it's failing with multiple factory loads. Plus, I'm trying to keep the variables down.
TheChunkNorris
11-19-19, 03:02
Yes. However, this rifle was manufactured when 55gr was the thing and it's failing with multiple factory loads. Plus, I'm trying to keep the variables down.
Fair enough.
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