Specs of the build are:

- Aero M5 upper and lower
- Criterion CL 16” .308 barrel
- Seekins Precision BCG

So to start off, I noticed some of the bolt lugs on the original Seekins bolt were exhibiting some pretty serious peening/gouging.  This was around the 500 round mark.  This damage was to both the front of the lugs and the rear.  Pics of the first bolt:

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Thinking this was just an issue with the heat treat of the bolt, I emailed Seekins and they promptly agreed and sent me out a new bolt.  Props to them on that, they were super responsive and helpful.

During this time period, I picked up a JP .308 bolt just in case the new Seekins bolt took some time to get to me.

Fast forward a few weeks, I have the new Seekins bolt installed, take the gun to the range, and fire off about 100 rounds.  Get home, tear the gun down, and the new bolt is showing the same peening/gouging of the bolt lugs, just less pronounced (likely due to the lower round count).  Pics of the second bolt:
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Just today, I install the JP bolt and rack the CH hard 15-20 times allowing the bolt to slam home.  Tear apart the gun and I’m already starting to see damage to the front of a few bolt lugs.  There doesn’t appear to be any damage to the rear of the lugs, but that seems to be because the rear of the lugs on the JP bolt are radiused.

So anyone have any ideas? It appears the damage to the front of the lugs is occurring due to making contact with the corner of one of the lugs in the barrel extension.  Problem is, I can’t recreate the issue while slowly hand cycling the gun, nor does there appear to be any visible damage to the barrel extension.

I’m almost to the point of just replacing the entire barrel and BCG, but there’s nothing in stock anywhere to do that with.

Does the extension and the upper have feed ramps?  If so, can you take a picture of the rear of the extension/feedramps looking through the upper?

That was my first thought, because I’m always extremely careful when assembling uppers to ensure extension alignment.  It’s about as centered as it can be in the upper.  Also, during install, the extension pin to upper fit was relatively tight.

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I even used some sharpie on the corner that seems to be causing the worst peening and none of it rubbed off while hand cycling.

That’s possible and was also a thought, but I don’t have another large frame carrier and I’ve had zero luck finding one in stock.

I did notice that the carrier has some play inside the upper receiver which could theoretically give the entire assembly enough room to shift vertically and cause the bolt lugs to kiss the extension, but that play pretty much goes away when the gun is assembled and a mag is inserted.

Barrel appears to be rotated slightly counterclockwise.

https://www.ar15.com/media/mediaFiles/294984/48B099E1-7989-4335-BA2B-71FA8CA0678F_jpe-1828740.JPG

Looking at this pic, the barrel extension seems to be rotated slightly counter-clockwise compared to the feed ramp cuts and gas tube hole in your upper.  Which would make sense with the marks on the rear of your bolt being on the left side of the lugs and happening during extraction and under some pressure.

Can pin path is fine and there is almost no wear to speak of in the track or cutout of the upper.

Barrel is a 16” with mid-length gas.  Utilizing an adjustable gas block that has been properly tuned shooting almost exclusively M118LR.

DPMS pattern with the standard Aero M5 carbine extension and a KAK 308 heavy buffer.

ETA:  I should also add, all 3 bolts passed a headspace check with quality gauges.

It may just be something with the angle/aspect of the picture, but I assure you it’s centered.  

Besides, if you look at the dark spot on the lug at the 3 o’clock of the pic, that it where I’m getting the most peening.  If the barrel were rotated clockwise, it would move that portion/corner of the extension lug further into the path of the bolt lug, not away from it.

ETA:  I should also clarify, the rear portion of the bolt lugs on the JP bolt are radiused, which don’t seem to be showing any signs of wear.  I’m more focusing on the peening to the front of the bolt lugs, which IS still occurring with the JP bolt.  That dark area on the 3 o’clock extension lug is where the bolt seems to be impacting while slamming into battery.

As to the damage to the rear of the lugs, 3 of the lugs on the Seekins bolts are showing this peening on the “left” side of the lug while 1 of the lugs is showing it on the “right” side.

If you're getting significant bolt peening from just hand cycling, something is seriously out of kilter.

I would also think that damage to the front corners of the bolt lugs could only happen as the bolt carrier is going forward into battery and the bolt lugs are hitting the barrel extension lugs.  I can't think of any other point in the cycle where there'd be a potential of damage in that spot.  Are the barrel extension lugs showing any damage, or any other evidence of strange dimensions or sharp burrs?

While holding the bolt back by hand, reach into the upper and try to force the bolt head to rotate or move forward / back in the carrier.  Can you jiggle the bolt head enough to force the lugs out of timing with the barrel extension when the bolt is cycled forward?

Had a first gen PA10 that did that. Lugs chipped from striking the opposing lugs just like yours. Sent it back. I do not know what was out of spec. PSA either reubuilt it or sent another upper.

It is something that slips by occasionally. I think Aero was even making the PA10 parts at the time.

THIS
I pulled my Dad's crappy DPMS 308 apart to install a free float handguard. Put it back together and the bolt wouldn't even try to close. Pulled it apart a few times before I realized the barrel could rotate with the nut loose. The pin is mushroomed and loose from the factory. I made it work, but it was a pain.

Your bolt lugs are hitting the barrel extension. It looks like it's not centered. If it is, then something else is off with the upper, carrier or bolts.

Somebody is bound to ask, but were you using a reaction rod?

Certainly appears that way to the human eye.  You can also measure the pixels and see the left ramp to the center extension is about 10-20% longer than the right ramp to the center extension as well as measure the barrel extension angle against the upper and see it is about 2 to 3 degrees off.  This isn't a perfect measurement because of image compression and camera perspective, but it would be consistent with peening.

are you running 1 o-ring, or 2 o-rings with your extractor?  if 2, try it with just 1.  although the damage is already done, would be hard to be conclusive.  as long as it still extracts satisfactorily...

There is one tiny small mark on that 3 o’clock extension lug but that’s all I can see.  I’ll check for play in the bolt in a little bit.

No, didn’t use an actual RR, utilized a MI URR, which prevents the upper from rotating and exerting pressure on the index pin.

I can go look again and take exact measurements, but I’m almost certain it’s centered.

Even if it wasn’t, rotating the barrel further clockwise would rotate the extension lugs further into the path of the bolt lug that’s already being beaten up.  Also would make me wonder why it’s only happening with 1 lug (at least with the peening to the front of the bolt lugs).

One. Can you expand on why you think this could be causing my issue?

i posted this a while ago in a couple other threads.  copy pasta, some more embellishment.  but you're already running just 1 o-ring.  so, moot point.  you might try polishing the front face of the extractor claw, so that it can glide over the cartridge rim smoothly, to snap in place more easily, reduce friction.  See Hellbender's guide.

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my 2 cents:
bolt rotates counter clock-wise CCW going into battery, CW when coming out of battery.
from the pics, impact is occurring on left rear of lug.  Since there's no chipping on the front corners of the bolt lug, I'm inclined to say that the bolt is entering the barrel extension lugs unimpeded.  kinda implies that the barrel extension lugs are timed good-enough in the receiver so bolt can enter the barrel extension smoothly.

so, possibly:
bottom lugs of bolt, going forward, pick up the top most round in the mag, push it forward up the feed ramp.  As the round enters the chamber and is centered, cartridge bottom face is seated in bolt, but due to strong extractor spring pressure, extractor not yet snapped over rim, and going into final battery lockup, bolt is being forced to rotate CCW in this condition, extractor claw still not yet snapped over cartridge base, back of bolt lugs impacts barrel extension lugs, then final inertia of lock up momentum, extractor is forced over the cartridge rim, and now bolt fully closes.  After having whacked the barrel extension lugs.

short version:  bolt trying to rotate but claw hasn't snapped over cartridge rim.

FWIW, nothing more.


when you manually load a round slowly, how much of a press (if any) is needed on the forward assist for the last 1/16" of bolt travel (extractor snapping over cartridge rim)?
when you manually extract a unfired case, does the bolt unlock smoothly?

hopefully, the FA on your M5 upper works, most of mine (M5 upper receivers from Aero) out-of-the-box don't, I end up having to replace the FA with a more truly mil-spec one from a trusted top-quality parts source.  luck of the draw and all that.  pawl doesn't bend over far enough to engage the carrier serrations.

Thanks for the explanation, and FWIW, the angled looking cut on the front of the bolt lug in my first pic should not be there, so at least one lug (at the front) is making contact with the barrel extension on its way into battery.  I’ve also done a closer inspection of the extension with better lighting, and you can definitely see unusual wear on that 3 o’clock extension lug, so that kind of confirms it.

I was able to find a 308 BCG in stock, so ordered it to see if something may be up with the engineering/design of the cam pin track with the Seekins carrier.

If I get no luck there, I think I may just pursue replacing the barrel.

The barrel extension may be out of spec.

This is what I’m leaning towards as being the cause.  I’ll test out the new carrier with one of the older bolts to see if the issue is still occurring, and if so I’ll probably tear down the upper and contact Criterion.

Late to the party, but I would:

Contact Criterion

Yeah dude, if it is happening on multiple bolts it’s probably a safe bet that the extension is a lemon. I’m sure Criterion will check it out and make it right.

I believe Criterion uses and recommends Fulton Armory and JP bolts in their shop when they headspace.

You can probably send that JP bolt in with it and they’ll hook it up.

My MSR10 exhibited the same gouging on the rear of the lugs.  I don't think this is happening during loading. A. I just don't see the necessary force being present to do so. B. The way the metal displaced lead me to believe it was  the extension lugs digging in right before the bolt unlocked. On mine the worst gouging was the lugs adjacent to the extractor. When I put in a new bolt, it also exhibited gouging, less so, but still present, again on those same lugs.  I attributed it to a bad extension. Not sure why the lugs adjacent to the extractor took the brunt of the damage however.

Well, the BCG I ordered appears to be back ordered, so placed an order with Fulton Armory for one of their BCGs.  Also decided to just bite the bullet and ordered a new barrel as well.

Hopefully this fixes the issue as I really like the gun.

Pretty classic case of the bolt hitting the extension lugs a little off. Based on the pictures I’d say the barrel is slightly canted but you’re saying it’s totally straight so that could mean there’s a machining defect in the barrel or something potentially up with where the BCG is sliding in the upper.

This is one of those things where trying to troubleshoot yourself will be a lot of trial and error/assembly & disassembly. Personally I’d swap the upper receiver and have Criterion replace the barrel for ya. Both of those things should solve the issue. I’d also replace the bolt at this point because the peening isn’t going to reduce the risk of a failure.

Good luck, hopefully you can get this fixed without anymore headache!