AR-10 Bolt head issue

[Winkfish]

Good Morning to all your AR-10 folks.

My buddy just got a new AR-10 and we were shooting it this weekend and noticed that the bolt head was staying forward in some instances and then when the bolt locked back on the last round it was in the forward position. So when he would insert a new magazine it would fail to feed correctly. If you pulled the charging handle all the way back and released it when this would occur it would load the round correctly and continue to fire the rounds in the magazine without issue.

Here are the spec on the rifle:
Aero enhanced upper and lower.
Aero bolt carrier group
JP silent capture spring for ar-10
A-2 length buffer tube
Triggertech diamond trigger
24" ballistic advantage barrel
chambered in 6.5 creed

I put the initial 60 rounds down the gun to get it broken in and zeroed for him. Gun ran fine for me. I ran the bolt carrier very very wet during this period.

This past weekend he tried to shoot it at a PRS match and had several failure to feed due to this. The event got rained out so I brought the rifle home, stripped it, cleaned it and really lubed up the bolt and carrier and I am attempting to diagnose what is going on.

What I was noticing is if I pulled the bolt back, released it and allowed it to lockup on an empty chamber, don't pull the trigger, then pulled it back again, the bolt head would hang up and the bolt catch would hit the bolt body and not the head. If I would have had a magazine in with rounds it would have produced a failure to feed upon releasing the bolt.

If I pull the trigger on the when the bolt is in battery it will then lock back the bolt it would function as expected and produce no failures to feed.

If I try and manipulate the bolt head in the carrier while it is out of the upper, it is fairly stiff when compared to a different bolt carrier I have on a 308 AR-10.

With the gun only having about a hundred rounds on it, does it need a longer break in, better lubrication something else going on?

Attached is an image of what it is doing when it produces a failure. Any thoughts and guidance would be greatly appreciated. Thank you

 

[P7M13]

My first suspicion would be buffer/spring/tube. The stiff BCG is a problem too.
Questions to investigate:
Stiff bolt? As in how much force required? Your bolt should move in the carrier with a finger's touch.
Do you have to pull hard on the charging handle to lock the bolt back? If so, check your buffer/spring dimensions. If not, how about the buffer weight & spring K factor? I am not familiar with JPs captured springs, though their bolts are the best, IMO.
How is the gas system? When he had the failures, was he using the same ammo as you?
Do you have adjustable gas? Set it wide open if so.
Finally, video him shooting. Limp shouldering does happen in ARs.

 

[Winkfish]

All the ammo has been the same factory Hornady 147gr ELDM.
The gun runs the ammo well, and yes there is a supurlative arms gas block. The ammo is dropping at the 4 o-clock position.

By it being stiff what I am referring to is when you have the bolt carrier in hand and you are trying to manually manipulate the bolt is where it is stiff trying to rotate it and push it back and forth. The area in question is that bolt head to bolt carrier interface.

There isn't any additional force required to pull the charging handle and that process does not seem to be an issue.

When the gun is shooting I am not seeing it short stroke or have any issues there. It appears that this only happens on manual manipulation.

 

[rpol98]

sounds like you're not able to manually pull the bolt back with the charging handle, and get the bolt face to lock up behind the bolt catch?

if so, then you have a mismatch between your buffer length, and buffer tube length. or the wrong spring and you're fully compressing the spring and the compressed spring length is keeping the buffer from retracting fully.

are you using an AR-15 rifle buffer with the A2 rifle buffer tube?

 

[Darryle]

Put a heavy buffer in it, H3, bolt cyclic speed is too fast, bolt catch is missing the bolt and grabbing the carrier.

The design of the upper only allows the bolt to move in a predictable pattern in relation to the location of the BCG and the cam pin.

Suppressed? If not, open the gas block fully, run wet until you can replace the buffer, and personally, I would replace the JP spring with a quality Mil-Spec buffer spring.

I run my large frame ARs wet, CLP spray works great for me, no more than the majority of us shoot our ARs, wet isn't an issue.

As mentioned, mismatched parts. Pull the spring and install the buffer and manually slide the BCG into the buffer tube and you should see about 5/16" - 3/8" between face of the bolt and bolt catch if you have the correct buffer tube and buffer.

 

[purpledragon]

What he said, above...... I always use graphite powder on my AR's no liquid stuff.....Just me!!

 

[Winkfish]

I futzed with it for a bit last night and I I am able to get the bolt to go back that 3/8" as you mentioned. I found that the bolt catch is very stiff to manipulate and does not always come all the way up unless you are really paying attention to it. It seems like a little of an interference fit issues with the coatings between the lower and the bolt catch. That will wear with time and maybe run better?

 

[Darryle]

It will possibly wear in, but it sounds like there is something else going on.

Who assembled everything?

That bolt catch should move freely with the only resistance being the spring and plunger.

I wonder if if has the right spring installed, I have seen it and even attempted it in a fit of drowsiness.

I would pull the bolt catch and investigate the resistance, replace the buffer and buffer spring with an H3 version from CMC, Spike's or even Primary Arms. I use the H3 buffer on all new builds I help others with and Slash's Heavy Buffers for my rifles.

 

[Citified]

The bolt catch should operate freely with no binding. It is simple to remove the pivot pin with an Allen wrench a make sure it is not sticking. A five minute job literally. Sometimes the bolt catch to receiver is a little too tight. Easily corrected.

 

[Zen Archery]

I would have him continue to run it soaked for at least the first 200 rounds. AR10's also throw a lot of gas an adjustable gas block will help control gas issues. Also avoid polymer mags stick to heavy steel mags.

 

[rpol98]

I futzed with it for a bit last night and I I am able to get the bolt to go back that 3/8" as you mentioned. I found that the bolt catch is very stiff to manipulate and does not always come all the way up unless you are really paying attention to it. It seems like a little of an interference fit issues with the coatings between the lower and the bolt catch. That will wear with time and maybe run better?

did your buddy use the Aero 308 M5-specific LPK, with the Aero M5 bolt catch? or is it some other bolt catch? like aftermarket, or AR-15? i have a loose Aero M5 bolt catch in my hand, looking at it, and it doesn't have that MIM dimple on the backside. just makes me wonder. Dimple, no-dimple, could also be due to manufacturing variance, batches, etc, though, so not conclusive.

also, Aero M5 lower uses a set-screw to retain the bolt catch, so should be easy enough to figure out why it doesn't move freely. Maybe your buddy stuffed the wrong spring in there.

 

[Citified]

In addition to the bolt catch, the bolt should move without binding in the carrier. If it does not, I would remove the firing pin retainer pin, the firing pin and the bolt cam pin and inspect the ring/rings on the bolt. There should be a little resistance to sliding due to the ring friction but should move fairly easily with your fingers. A little lube in the bore during break-in will help but does generate carbon build up. Hope this helps

 

[TR1Hemi]

If the maker of the bolt catch is not Aero. Look at interference between the bolt catch and the inside of the upper when it is installed on the lower with an empty mag. If the bolt catch moves further without the upper on, the bolt catch is hitting the inside of the upper. This crap happens when "building" AR10's as there is NO MILSPEC. The trigger group is AR15 mil-spec much much of the LPK is AR10 specific, and since there is no MILSPEC parts differ based on the UPPER. I had this exact issue with an 80% lower and PSA upper, I ground the bolt catch a bit where it was hitting the inside of the upper.

 

[Darryle]

Yes, very good points brought up.

The standardization of specs across the large frame AR is one thing I wish would happen. Aero is forcing some to up their game, there was a time when some uppers would not work with the Aero M5 lowers. The cost of these is flooding the market with excellent quality lowers and others are altering their uppers and other parts slightly to work fluidly with the M5 lowers.

If you didn't disassemble the BCG and thoroughly clean and lube it, I would suggest that now, inspect the bolt bore for wear, there should be obvious "shiny" spots, make sure these spots are concentric inside the bore, also verify that the gas rings are indexed correctly to each other.

Worse comes to worst, I have a ToolCraft HP, .062" FP dual ejector complete BCG that I am willing to clean up and loan to you to remove that potential problem or help diagnose. I also have a Carbine H3 large frame buffer I can add to the loan at the same time.

Keep us informed, but I think you have received some excellent advice to try out next range trip to see if you can identify the issues.