Reliability problem with AC556F

[Bret]

Whoever had my older one before disassembled it without removing the pistol grip, but they buggered up the pistol grip some in the process. I'm just playing it safe. I didn't check the headspace, but I can use my comparator to measure the spent cases.

 

[Bret]

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.

 

[anm2_man]

Looking at your pictures that is a REAL failure to eject. Is the bolt the bolt trying to pick up the next cartridge out of the magazine and it hasn't removed the spent cartridge ? My guess would be either a chamber problem or a positioning issue with the magazine. I also assume you have tried switching magazines already ?

Since you have 2 guns, you need to get the problem to move. I would probably try swapping the trigger groups and if the problem doesn't move, then swap the stocks ?

Beyond the above, I'm out of ideas.

 

[Bret]

Is the bolt the bolt trying to pick up the next cartridge out of the magazine and it hasn't removed the spent cartridge ?

Yes

I also assume you have tried switching magazines already?

That was the first thing that I tried. Every magazine runs 100% in the old rifle, but not in the new rifle.

Since you have 2 guns, you need to get the problem to move. I would probably try swapping the trigger groups and if the problem doesn't move, then swap the stocks ?

That sounds like a logical next step.

 

[anm2_man]

Me To - Keep us posted.

 

[Gemlin]

Mine did something similar. I shaved the mating surface on the extractor that mates against the bolt. This will cause the extractor to hold the brass tighter. This slight mod fixed my gun.

I also had a spare 187 series mini bolt that would run 100% in my AC556 so I knew it was something with my original bolt. My spare bolt head spaces correctly in my AC556

 

[Bret]

I shaved the mating surface on the extractor that mates against the bolt. This will cause the extractor to hold the brass tighter.

So do you think that the problem was with the extractor or bolt? In other words, did you adjust the extractor to compensate for something being out of spec with the bolt?

Also, how did you determine that the spare bolt had the correct headspace for your rifle? I'm tempted to swap them, shoot a few and measure the headspace of the spent cartridges using my headspace comparator. However, I'm worried that if it was off significantly, the thing would grenade.

 

[Gemlin]

I have head space gauges. I've tested them with my AC556 and 187 series mini. They all headspace the same.

Not sure if my bolt was out of tolerance or not for the extractor mounting or ruger changed the extractor slightly. After wearing out the original I put a replacement one in and started having issues. I shaved the extractor slightly and installed it and all issues went away.

I bought a spare 187 bolt off ebay and checked it in my gun and head spaced fine. Closes on GO doesnt close on NO GO.

My replacement 187 bolt runs 100%. My original bolt with shaved extractor runs 100%

Odds are your bolts are interchangeable. I personally havent seen one that hasnt been. At least with my small sample of bolts. (Ive tried 4 and all head spaced fine)

You can buy a set of gauges for under 50. You just need to put the gauge in the bolt into the extractor and close it by hand. Dont let it autoload it.

 

[Bret]

I have a set of gauges. I should have checked the headspace of each bolt in the opposite rifle when I had the bolts stripped.

 

[Gemlin]

You can do it with assembled bolts

 

[Bret]

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.

 

[sniperdoc]

5.56 and .223 require different headspace gauges.

 

[Bret]

Right, but it is call an AC556F. I wonder what the difference between the two is. Regardless, I'm still having issues when using factory 223Rem.

 

[sniperdoc]

Right, but it is call an AC556F. I wonder what the difference between the two is. Regardless, I'm still having issues when using factory 223Rem.

Slight dimensional differences, almost all firearms will chamber both, but gauges made for one will not always give accurate readings in a chamber cut for the other.
What caliber is marked on your barrel? Use only that caliber GO/NO GO gauges when checking the chamber.

Also, look in your chamber and throat and see if it has any scratches, rough or pitted areas or rust. If you don't see any, take a Cotton Swab and rub it all over the chamber walls and throat. If there's anything you didn't see, the strands of cotton will catch and you may feel it hamg or see strands stuck in your chamber/throat afterwards.
Also, your chamber and throat should be polished literally mirror smooth. If not, it may simply need a polishing job.

EDIT: I was finally able to pull up the pics you posted including the case heads. You REALLY need to disassemble the bolt and scrub the crap out of the Bolt Face including under the "lip" (Dental Picks work well for this) (polish the Bolt Face while you're at it), Ejector, and Extractor (polish it too) and run a Cotton Swab all over it as above to find any burrs.
I'd also recommend scrubbing the entire gas system, including the Piston and Tube with Carburetor Cleaner, and cleaning the gas ports (Dental Picks, Pipe Cleaners, and Carburetor Cleaner,etc)

Have you checked fired Cases with a Micrometer vs unfired Cases?

 

[Bret]

Have you checked fired Cases with a Micrometer vs unfired Cases?

I've checked them with dial calipers and the headspace comparator. What would I be looking for with a micrometer?

I'll try the cotton swab idea. It's nice and simple. Why didn't I think of that?

 

[sniperdoc]

You're looking for any indication that the Chamber is Out-of-Spec (bulges in the case, case lengthening,etc)
The Cotton Swab is an old Gunsmithing Trick I learned from my Mentor.