Reliability problem with AC556F

[Bret]

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 since the former owner purchased it. 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 seem 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.

Is there a torque specification for these allen head screws?

 

[parrthed]

I’d give it a good clean and lube and see where that gets you. Were there any buffers installed?

 

[Bret]

None. It looks identical to my other AC556F.

 

[Mackjack]

You got approved forms in 9 months? Nice!

 

[Bret]

I don't think so. How long does it take them to do the background check? It's less than a day. The wait should be just a few days if that. The rest is just the government justifying its existence.

Any idea where I can get the torque specification for the allen head screws?

 

[Offmarksman]

Have you tried replacing the ejector?

 

[Bret]

No, but I'll examine both ejectors to determine if I can see a difference. I guess it could be the ejector spring too? That does beg another question. Can I swap bolts assemblies between both rifles without creating a headspace issue (like an AR15 or M16) or is the headspace set to the bolt (like an AK)?

 

[slimshady]

I don't think so. How long does it take them to do the background check? It's less than a day. The wait should be just a few days if that. The rest is just the government justifying its existence.

Any idea where I can get the torque specification for the allen head screws?

AFAIK Ruger has never released a torque spec, but the standard engineering spec for that size screw is 32 inch pounds. In addition, I believe they whack the bottom of the screw with a center punch to expand the end and stake it in place. Which is why new screws are recommended. You can also get various sized "gas bushings" to tune your cycling from the aftermarket.

Modern technology is often helpful when diagnosing cycling problems. If you have a camera that does movies, setting it to low res and max frame rate can make it into a pseudo-high speed camera to film the gun firing. Slow it down on playback and you can maybe see exactly what is happening.

From what I have heard, a major slowdown on transfers is the running of the fingerprints. ATF hands them off to the FBI for that process, and they are the lowest priority. So if the FBI is busy, it may take a while for the report to come back. That, plus there are only a dozen or so examiners that process ALL the NFA paperwork. In addition to the normal transfers, they also have to deal with LE stuff. The Feds are exempt, but state and local LE still have to transfer everything NFA, they just do so tax-free. One assumes the background check is also done away with, but the data still has to be entered into the NFTR. And every flashbang grenade they have has to be transferred to them, and then removed when it is used. Between training and actual use in the field that's probably a full time job for someone. What would really speed things up is ATF getting their own fingerprint system, and doing multiple transfers as one instead of duplicating everything. And hiring more examiners!

No, but I'll examine both ejectors to determine if I can see a difference. I guess it could be the ejector spring too? That does beg another question. Can I swap bolts assemblies between both rifles without creating a headspace issue (like an AR15 or M16) or is the headspace set to the bolt (like an AK)?

I believe they are individually fitted to the gun, that's one of the parts Ruger won't sell, you have to send them the gun and they fit it for you.

 

[Bret]

I believe they are individually fitted to the gun, that's one of the parts Ruger won't sell, you have to send them the gun and they fit it for you.

That makes sense given that they won't sell them to you. Unfortunately, they won't work on the gun.

 

[Bret]

I checked the ejector of each AC556F. They both seem to apply firm pressure and are comparable.

 

[anm2_man]

First off, I believe this gun has had this problem for a long time, because there is no reason to fully disassemble the gas system which requires removing the 4 screws. As Slimshady stated, they are staked and you need to buy new ones before reassembly. Sounds like who ever took it apart, didn't buy new screws.

Since you already checked the function of the extractor, I would order a new Main spring from Wolf Springs. The AC556 uses the mini-14 Ranch rifle spring and Wolf should have them in stock. Here is a link to the springs NOTE ORDER THE 181 Spring IF THE AC556 is a 191 series - https://www.gunsprings.com/RUGER/MINI%2014,%20MINI%2030,%20RANCH%20RIFLE/cID2/mID52/dID222. Since you know (Just like everybody else) little about the history of that gun, or the history of the main spring. If there is a minor kink in that spring, it could cause the resistance condition you mention in your description. Further on that subject (since you have a full functioning AC556), I would swap the OP Rod from your old gun into the new gun, and then re-check the pull to see if that resistance goes away (I would also do this before replacing the main spring). If the resistance goes away, Finding an OP Rod could be a challenge, but not insurmountable (A OP Rod from a older 181 series Mini-14 could be a replacement).

Hope that info helps.

 

[Bret]

I took a look at the staking on both rifles. You almost need a magnifying glass to see it. I'll get some more screws.

Both rifles are 191 series. 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.

 

[Bret]

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".

 

[slimshady]

"We set the torque spec here at the factory with air pressure." Translation: The air screwdriver selector is turned to "2". Expansion does make sense, so I would simply tighten them up snug, then maybe another 1/8 turn to put a little tension on them. Staking is easy, all you need is an automatic center punch. The $4 Harbor freight model should work to do four screws.

https://www.harborfreight.com/spring-loaded-center-punch-621.html

 

[Bret]

OK, I have a couple of those. Now that I think about it, the existing staking looks like it might have been done with something similar.

 

[Offmarksman]

I still think it’s the extractor or spring. Have you taken it out and cleaned and examined the parts? PM me if you are looking for an older op rod.

 

[Bret]

I think that you're likely right, but have not had a chance to examine it further.

 

[Bret]

I sent pictures to Ruger and received another reply from the same helpful gunsmith. He also 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.

 

[Bret]

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.

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.

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.

 

[KickStand]

^
I’ve got three of these stocks on ac556k and two mini 14’s.
I do NOT have to take the pistol grip off on any of them to remove the stock. I have a small flat head that I can easily fit into two of them and remove the stock. One of them is a lot tighter / closer than the others. I think Ruger changed the pistol grip slightly over the years to make it easier to remove the stock.

I really hope changing the internals of the bolt will fix your issue. Like someone mentioned before, your newer ac556k probably had problems for awhile.

Any chance you checked the headspace?