6.5 Grendel bolt breakage

[searcher]

I assembled a 6.5 Grendel with a new barrel that was completely machined, chambered and matched to a bolt by a very well known and respected barrel manufacturer. The bolt used was from another very well known manufacturer of quality products. I have assembled many A/R's in different calibers but this is my first in the Grendel. I had fired around 1,000 rounds of light to full-power factory-ammo velocities. At this time, the bolt lugs on either side of the extractor broke off. I sent the bolt back to the outfit I purchased the barrel from and they replaced it. While waiting for the replacement, I saw that another company had obtained some bolts so I bought one for a spare. It came first and I installed it after checking for allowable headspace. The replacement bolt cracked in the same place after only 200 rounds fired - and these were relatively mild rounds. I have tens of thousands of rounds on many A/R's in other calibers and never had a catastrophic bolt failure before. Now I have two in a very short span - leading me to think something may be amiss with the bolt lug contact or maybe I was just very unlucky. I tend to think there is a problem. I am picky about my cleaning and lubrication - and I am shooting ammo that is properly "headspaced" to the chamber with about .004" clearance. Has anyone run into such failures - and what should I be looking at? I don't want to waste another bolt if I am overlooking something.

 

[chuckbuster243]

Check to see how square the receiver face is in relation to bore. Could be crooked enough to cause load on one side more than the other maybe? Get a lapping bar and face it if needed. Also was the replacement 9310 steel? Maxim Firearms in lake Havasu is where I have been getting mine. Or also check or replace the extension... Something is up if its popping after 200.. Matt

 

[searcher]

I'm not sure what kind of steel was used in the bolts. The first bolt was a Les Baer. I was reluctant to use the name as having two such breakages with two different manufacturers - I didn't want to point blame to a good company. Likewise, the barrel and extension were from Krieger, whom I've bought many barrels from and was never less than completely satisfied. I too suspect the barrel extension not being square to the lugs. There may just be a burr which has formed that I cannot readily see. Time to break out the bore scope after thinking about this a bit more. I appreciate the info and am going to look for a lug lapping tool. I guess I'll start with Brownell's as I had bought a receiver face lapping tool from them a while back. Thank you

 

[gunsandgunsmithing]

I assembled a 6.5 Grendel with a new barrel that was completely machined, chambered and matched to a bolt by a very well known and respected barrel manufacturer. The bolt used was from another very well known manufacturer of quality products. I have assembled many A/R's in different calibers but this is my first in the Grendel. I had fired around 1,000 rounds of light to full-power factory-ammo velocities. At this time, the bolt lugs on either side of the extractor broke off. I sent the bolt back to the outfit I purchased the barrel from and they replaced it. While waiting for the replacement, I saw that another company had obtained some bolts so I bought one for a spare. It came first and I installed it after checking for allowable headspace. The replacement bolt cracked in the same place after only 200 rounds fired - and these were relatively mild rounds. I have tens of thousands of rounds on many A/R's in other calibers and never had a catastrophic bolt failure before. Now I have two in a very short span - leading me to think something may be amiss with the bolt lug contact or maybe I was just very unlucky. I tend to think there is a problem. I am picky about my cleaning and lubrication - and I am shooting ammo that is properly "headspaced" to the chamber with about .004" clearance. Has anyone run into such failures - and what should I be looking at? I don't want to waste another bolt if I am overlooking something.

That's where AR bolts break..on either or both sides of the extractor, first. The Grendel has a lot more bolt thrust than a .223. I suspect warm loads and high temps, along with the AR platform being the causes.
Are you chronoing the loads?

 

[searcher]

Yes, I chronographed all loads worked up. Admittedly, some of those fired with the first bolt were quite warm - but not beyond published data. Those fired with the second bolt were all the same load which were well under factory velocities using same bullet. Perhaps the powder I used was too quick for the bullet used, despite using published data. I know the boltface looks pretty "weak" compared to the 5.56, etc. I guess I'll switch powders after further inspection just to be sure. I did find that is is much more difficult to determine initial signs of overpressure due to the "massively" constructed Lapua cases used.

Does anyone know whether bolt failure is much more prevalent in this caliber over, say, a 5.56 ?

 

[gunsandgunsmithing]

Yes, I chronographed all loads worked up. Admittedly, some of those fired with the first bolt were quite warm - but not beyond published data. Those fired with the second bolt were all the same load which were well under factory velocities using same bullet. Perhaps the powder I used was too quick for the bullet used, despite using published data. I know the boltface looks pretty "weak" compared to the 5.56, etc. I guess I'll switch powders after further inspection just to be sure. I did find that is is much more difficult to determine initial signs of overpressure due to the "massively" constructed Lapua cases used.

Does anyone know whether bolt failure is much more prevalent in this caliber over, say, a 5.56 ?

I thought I answered that....At the same chamber pressures, the bolt thrust IS much higher, so yes, they can break easier than 223/5.56 bolts.


That said, faster powders compound the problem in an AR platform. I can break the two lugs you broke in one shot without having chamber pressure issues.


What is your load?

 

[jbarron]

There is an improved bolt for the grendel. The normal 5.56 bolt will not hold up.
Check Alexander arms for a bolt. Unfortunately I think it will affect headspace.

 

[gunsandgunsmithing]

There is an improved bolt for the grendel. The normal 5.56 bolt will not hold up.
Check Alexander arms for a bolt. Unfortunately I think it will affect headspace.

The AA bolts do seem to hold up better than the less expensive 7.62x39 and ppc bolts but are .011" deeper. So you are corrrect, they do change headspace vs. those bolts. I'm not sure what Les Baer uses in his. Definitely, confirm hs if changing bolts.

 

[searcher]

Thanks for the info!

 

[searcher]

I thought I'd add what my experience was with the AA bolt. I bought an Alexander Arms complete upper from a friend who fired just a few boxes of ammo through it. I put another 1,200+ rounds through it before selling the entire upper due to wanting a heavier and longer barrel - and perhaps a bit more accuracy. Had NO problems with it.

 

[Rtheurer]

Broke one lug off a 6mm Version of the Grendal last summer.It was a used upper so im not sure how many rounds Exactly where on it.
Robert Whitley is selling some really nice Hardend bolts for this aplication that seems to be holding up exceptionaly well from what I have been told by other shooters.
I also have on a second upper an AA bolt, its been doing real well also.

This is not unnusual as I have found out doing some research along the way.

RussT