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 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? Any help is definitely appreciated.

This gun has a fixed piston gas system, maybe it needs cleaning or lubrication?

I field stripped it, cleaned it (no shooting residue found), lubricated it and reassembled it prior to shooting it.

Did the piston move freely?

Yes, this 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. Based on the carbon buildup, it looks like some gas may have been leaking out. I'll soak all the parts and clean everything out.


Is there a torque specification for these allen head screws?

I field stripped my first AC556F so that I could compare the two. As you can see from the picture below, it has significantly more carbon residue. Of course that's where the gas comes out and hits the piston, so that's to be expected. There's no way to tell with this one if there's any leakage around the front sight gas block. I'm not going to unscrew the allen head screws because this one is working correctly and I don't want to mess it up. Another thing to note is that the recoil spring for the 2nd one is about an inch shorter than the 2nd one. Therefore, I'm betting that installing a new recoil spring in the 2nd one would only make the problem worse.

I would not just rule out the spring is OK. It is shorter than the spring in the gun that works, right? Put the 2nd gun back together with the recoil spring of the 1st gun, cost is nothing. It may work. BTW, if the 2nd gun was stored for long time with the action locked open, that can and or will show the spring to be shorter and maybe in need of replacement. I had that happen to a used BAR where the owner stored it for years locked open. It was a 1 shot semi auto until I purchased a new spring. You have likely read about springs taking a set, just saying that is a easy fix and easy to check since you have 2 springs.

I have new recoil springs, so I can give it a try. I'm just thinking that if it's short cycling, the new recoil spring is only going to make it worse. The short distances that it's throwing the spent cases makes me think that it's not getting enough gas. If the action is cycling too fast and/or it's unlocking too early, then the a new spring might help. I'll clean, reassemble and give it a try. First though, I need to find out the torque specification for the allen head screws.

Bret said:

First though, I need to find out the torque specification for the allen head screws.

Click to expand...

Agree. Also check the tiny gas bushing that transfers gas from barrel to piston. Make sure it's not clogged...make sure it is there, they are easily lost. "Once the gas block has been separated, be careful that the little gas port bushing doesn't get away from you. This is a tiny little tube that transfers the gas escaping through the gas hole in the barrel into the gas block and through the gas piston to work the action."

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.

Check the extractor and ejector.

Three weeks ago I swapped all the bolt components between the rifles. It didn't make a bit of a difference. The old AC556F ran 100% and the new one still had the same extraction, ejection and hesitation issues.

I then 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.

Ammo issue? Does the full auto feature require "full auto" ammo? 8)

No. I've tried both factory loaded 223Rem and factory loaded 5.56x45 without a difference in the results.