Well that’s unfortunate.....

[Tarowah]

I was planning on hitting the 600 yard range today so I pulled the Grendel out of my bag to run a bore snake through the barrel and get everything lubed so I’d be ready to hook and book this morning.

When I was finished shooting on the last range trip I didn’t notice anything out of norm, the rifle shot fine with zero issues.

When I pulled the charging handle last night it was a little sticky which I thought was strange so I gave it a good tug, double checked that the chamber was clear and commenced to field stripping, but when I pulled the bolt out, DAMNNNNN it..... lol

It’s the bolt that came with the Brownells/Satern Grendel barrel, approximately 1400-1600 rounds (give or take) through the gun.

 

[GymB]

You’ve got 6 lugs left, what could go wrong?

The wear pattern is interesting.

 

[Tarowah]

You’ve got 6 lugs left, what could go wrong?

The wear pattern is interesting.

My wife said “is that dangerous?” And I said “nah, as long as the rest of them hold it shouldn’t blow my face off”, she wasn’t amused haha

 

[JBoyette]

I was planning on hitting the 600 yard range today so I pulled the Grendel out of my bag to run a bore snake through the barrel and get everything lubed so I’d be ready to hook and book this morning.

Be advised, its the extractors fault. The radius is incorrect. Be careful on any extractors used. Be sure its not a 7.62x39 extractor.

If you are buying a bolt, get it from JP. Its Its worth the cost. OR buy a bolt from Tbox barrels

Do you have any of your once shot brass?

 

[Tarowah]

Be advised, its the extractors fault. The radius is incorrect. Be careful on any extractors used. Be sure its not a 7.62x39 extractor.

If you are buying a bolt, get it from JP. Its Its worth the cost. OR buy a bolt from Tbox barrels

Do you have any of your once shot brass?

I actually saved a couple of pieces of once fired brass and put them aside, I knew Grendel bolts can be problematic and read it was a good idea to keep at least one in case of bolt failure, the idea is I could use it to “hand check” a new bolt in the event of a failure and I don’t have a go/no go gage.

I was actually looking at a JP or AA bolt and a buddy of Facebook contacted me this morning and said he had an extra bolt I could have, the barrel is a “Type II” and his bolt is a .136 so I should be good to go once I check headspace.

Edit: Also you’d think a bolt sold with a barrel would be properly machined, I guess you do get what you pay for!!

 

[Get Off My Lawn]

Be advised, its the extractors fault. The radius is incorrect. Be careful on any extractors used. Be sure its not a 7.62x39 extractor.

If you are buying a bolt, get it from JP. Its Its worth the cost. OR buy a bolt from Tbox barrels

Do you have any of your once shot brass?

Any thoughts on the AA bolt? That’s what I have.

 

[JBoyette]

@Tarowah and @Pink_Vapor

I do not know bolts by brand.

BUT notice what I typed. Its the extractor that caused the bolt to fail. Look at the image posted, extractors radius and then look at the bolts radius. Its not the same radius. This causes the edges of the extractor act like fangs and cause a sticking point on the unlock, The fangs cause 100% lockup that forces the brass to rotate in the chamber no matter the stage of the brass expansion. Keep in mind that when a AR bolt unlocks before extraction, the brass does not HAVE TO rotate in the chamber and is not suppose too if the brass has not contracted.

Keep in mind, this is why high pressure loads have a shinny ring on the brass case and show the ejector plug when it "connects". And also whey you can have dirty necks.

 

[GymB]

Since you guys are ignoring me I’ll be more specific.

Looking at the wear to the finish to the lugs in the top pic. It seems that the face of the lugs at 3, 9, 11, and 1 are making contact, but the others are not. The wear makes it seem that the force is greatest right were the 11 lug should have been.

Is it possible that the BCG isn’t square to the barrel extension, or is this just normal, I have limited experience with the platform.

 

[JBoyette]

Since you guys are ignoring me I’ll be more specific.

Looking at the wear to the finish to the lugs in the top pic. It seems that the face of the lugs at 3, 9, 11, and 1 are making contact, but the others are not. The wear makes it seem that the force is greatest right were the 11 lug should have been.

Is it possible that the BCG isn’t square to the barrel extension, or is this just normal, I have limited experience with the platform.

The wear pattern supports my idea of the extractor. During the unlock phase, the extractor has a hold of the brass, the bolt carrier moves rearward, the cam pin forces the bolt to rotate, the extractor "fangs" are holding onto the brass, the brass and bolt rotate. The load is placed on the nearest bolts to the extractor.

BUT remember the bolt spec is to the extension lugs not the breach face. The wear marks you are talking about is on the breach face. The other side of the lugs are making contact with the extensions lugs and going from locked to unlocked.

 

[Tarowah]

Since you guys are ignoring me I’ll be more specific.

Looking at the wear to the finish to the lugs in the top pic. It seems that the face of the lugs at 3, 9, 11, and 1 are making contact, but the others are not. The wear makes it seem that the force is greatest right were the 11 lug should have been.

Is it possible that the BCG isn’t square to the barrel extension, or is this just normal, I have limited experience with the platform.

Apologies JimB, I was in a similar situation not knowing if the wear patterns were normal and John just covered that, His explanation makes perfect sense now that I see the wear patterns and see the extractor issues with on the bolt.

 

[GymB]

The wear pattern supports my idea of the extractor. During the unlock phase, the extractor has a hold of the brass, the bolt carrier moves rearward, the cam pin forces the bolt to rotate, the extractor "fangs" are holding onto the brass, the brass and bolt rotate. The load is placed on the nearest bolts to the extractor.

BUT remember the bolt spec is to the extinction logs not the breach face. The wear marks you are talking about is on the breach face. The other side of the lugs are making contact with the extensions lugs and going from locked to unlocked.

I have a language problem, but you see what I’m talking about. However it seems to me that upon firing the bolt group will be moving backwards, so there will not be anything for those spots to rub against even if the extractor is hanging onto the brass and forcing it to rotate.

Again I most likely have it wrong, but do me a favor and give some thought to how it could be happening during the loading cycle. I was thinking that the leading face of the upper lugs is banging into something while loading. I don’t think it should be, but I haven’t a rifle apart in front of me.

 

[GymB]

Apologies JimB, I was in a similar situation not knowing if the wear patterns were normal and John just covered that, His explanation makes perfect sense now that I see the wear patterns and see the extractor issues with on the bolt.

No apology needed, was just poking fun. I’m not yet following his explanation, but I’ll get there, probably just need to get the wrong ideas out of my head first.

 

[JBoyette]

I have a language problem, but you see what I’m talking about. However it seems to me that upon firing the bolt group will be moving backwards, so there will not be anything for those spots to rub against even if the extractor is hanging onto the brass and forcing it to rotate.

Again I most likely have it wrong, but do me a favor and give some thought to how it could be happening during the loading cycle. I was thinking that the leading face of the upper lugs is banging into something while loading. I don’t think it should be, but I haven’t a rifle apart in front of me.

Jim,

On your AR remove the BCG. Take a thick tiped Sharpie and color both sides of your bolt lugs.

Reinstall the BCG.

Hand cycle the action with the charging handle like 15 times.

Remove the BCG and inspect where the ink is removed.

That will show you the side of lockup

 

[GymB]

Jim,

On your AR remove the BCG. Take a thick tiped Sharpie and color both sides of your bolt lugs.

Reinstall the BCG.

Hand cycle the action with the charging handle like 15 times.

Remove the BCG and inspect where the ink is removed.

That will show you the side of lockup

I understand that, the mechanics of the lug engagement makes perfect sense. But, I’m going to do what you suggest anyway to see what shows up. What I believe is that when the gun is fired the cartridge pushes back on the bolt creating pressure between the back of the bolt locking lugs and the front of the lugs in barrel extension, as the bullet travels past the gas block, gas pressure pushes the gas key and bolt carrier rearward, rotating the bolt. None of that creates wear on the front of the bolt lugs, it’s all happening at the back of the bolt lugs, which is what I think you’re pointing out.

My question is perhaps so ignorant as to be almost incomprehensible, but let me try again, and it’s about the wear on the non-locking side of the bolt lugs, I call those the front side of the bolt lugs.

I believe that the front side of those lugs wear in only two ways, both of which happen before the gun fires. Firstly the lugs at 9 and 11 get worn from sliding against the brass getting picked up from the magazine. Secondly chambering ends when the lugs strike the rear of the barrel which stops the forward movement of the bolt, and as the bolt carrier continues forward the bolt rotates, so there will be wear on the front side of the locking lugs where they impact and then rub against the barrel during chambering. Does that sound right? Is there any other action that wears the front of the bolt lugs?

I expect the wear from picking up the brass from the magazine to be minor and the wear from the bolt striking the barrel to be uniform. In the first picture the wear is clearly not uniform, and the most wear is right where the broken lug was, that’s what I think is interesting and I’m asking if it is indicative of a problem.

 

[Amps 13]

Well there’s your problem!

 

[Flashpoint]

What John says makes sense to me, since the extractor is a separate piece and can move slightly in its channel, it can transfer the majority of the rotational load to the closest lug in the direction of rotation.

What I don't get is how that rotational load affects the uneven wear that is shown on the front of the lugs. It sure looks to me like something is not square like JimB has pointed out, like the missing lug was hitting the barrel face harder than the others.

 

[GymB]

What John says makes sense to me, since the extractor is a separate piece and can move slightly in its channel, it can transfer the majority of the rotational load to the closest lug in the direction of rotation.

What I don't get is how that rotational load affects the uneven wear that is shown on the front of the lugs. It sure looks to me like something is not square like JimB has pointed out, like the missing lug was hitting the barrel face harder than the others.

Honestly I hadn’t thought about that rotation putting strain on the adjacent lugs, which is a bit humbling. I would expect that the lug at 1:00 would take the worst of that, but I can see where either could be broken.

I still think that something is not square. Could be a bugger on the back side of that lug that broke off or on the mating lug in the barrel extension, either would force the top of the bolt too far forward in battery. Could be that the inside of the barrel extension isn’t square to the barrel, causing the same problem. Could be something else, or could be that it’s just normal. I obviously don’t know, but I’d check for unusual wear in the barrel extension and measure the gap from the barrel to each lug in the barrel extension just to see if it reveals anything.

 

[JBoyette]

One of the mental dances we have to do is understand that that in unlock the extractor is inline with the ejection port.
going to lock the bolt rotates counter clockwise.
Going to unlock the bolt rotates clockwise.
Keep in mind this is for a right eject system.

So I turned the given picture to represent the proper view looking at the bolt from down the barrel per-say.




This now shows one lugs wear could be from stripping from the magazine.

 

[Sigequinox220]

Honestly I hadn’t thought about that rotation putting strain on the adjacent lugs, which is a bit humbling. I would expect that the lug at 1:00 would take the worst of that, but I can see where either could be broken.

I still think that something is not square. Could be a bugger on the back side of that lug that broke off or on the mating lug in the barrel extension, either would force the top of the bolt too far forward in battery. Could be that the inside of the barrel extension isn’t square to the barrel, causing the same problem. Could be something else, or could be that it’s just normal. I obviously don’t know, but I’d check for unusual wear in the barrel extension and measure the gap from the barrel to each lug in the barrel extension just to see if it reveals anything.

I very clearly see what you been referring to.

if we # the lugs 1-7 moving clockwise, then #7 no longer exists (sheared off). But, for some reason UNexplained by any rearward bolt motion/forces, lugs #3/4/5 look factory new finish. Bolt 6 in particularly friction worn and presumably missing #7 would look similar or worse. #1 seems to have reduced wear and #2 less wear still.

very asymmetrical front face wear on the lugs which rear motion does not, to your (or my) knowledge, provide adequate explanation.

bizarre. Ultra rare bolt comorbidity with previous theory or the real culprit? The mystery deepens....

 

[LesBaer45]

There was a run of Satern 6.5G barrels that had the chamber cut too short. Might be worth it to check.