First Bolt Failure

[LRRPF52]

I've only had this happen to me with 5.56 blasters that I've run hard with CLGS shooting a mix of M855 and Wolf, with 2.9oz buffer. That was after over 10,000rds of tortuous high volume sessions, competitions, and training.

I haven't held back on any of my Grendels, but I don't shoot them in the same kind of volume either since I'm usually doing more distance work.

If you break a bolt with Wolf in a MLGS, something is wrong somewhere with materials and/or other components/assembly method.

I would second the upper receiver being square check. This is in the TDP, but a lot of manufacturers have been unaware of it in the past until more recently. The anodizing is where the square face usually gets off due to non uniformity in the process if it isn't controlled well by the anodizer.

[grayfox]

where is that metallurgist when we need him? From the picture it's hard to tell but looks a little like an MIM failure. Failed from the rear... possibly an early extraction attempt... wonder what the corresponding extension lug looks like.
All just brainstorming however.

[A5BLASTER]

lets see if i can post that pic for ya

OP. Could you take the bolt out and clean it. So we can get a better look at it next to the bcg.

[lazyengineer]

Thanks for posting this - always useful to see and learn from failures. For a gun and cartridge supposedly considered for military adoption, it damned well better be able to keep up with that rate of fire. I've taken my Grendel 3-gun shooting, and there are video's from Grendel promoters showing full-auto fire. At 52,000 PSI rating, it should be able to take that all day.

For what it's worth, Wolf broke my Grendel extractor last week, on my PSA rig - at just 600 rounds in. 1st shot of the day. That rig has a carbine gas system. And carbine gas systems see a harder life in general. Combine that with Wolf steel, which is loaded on the high end of that (I think), and is steel, and things see a harder life. Still, it shouldn't have broke, and hopefully they will RMA that bolt for you.

In the meantime, I did find one place that has reasonably priced bolts and extractors. PSA supposedly will replace my broken extractor, but haven't yet. So I picked up a couple from Black Rifle Arms, which were under $20. They are selling bolts for $70. For all I know, it's junk; but heck - can't be any worse than the part that broke already.

[rwh]

I've broken three bolts so far. There are plenty of bolts around. I got the one I'm shooting now from a group buy on this site and it's at about 3000 rounds knock on wood. Get another bolt and keep a spare. It's not a big deal but watch the heat on the handloads.

This one looks like it will do: https://faxonfirearms.com/faxon-ar-15-6-...0-nitride/

[Fess]

Mdram, does that bolt have a coating of some kind?

[HuntTXhogs]

I read on another forums that nitride bolts could be an issue

Just the messenger

[Fess]

I read on another forums that nitride bolts could be an issue

Just the messenger

That is what I was thinking. One of the concerns is that the temperature used for nitriding is close enough to the steel's annealing temperature that it could affect the properties of the bolt. I have not seen any hard data, though.

[grayfox]

Rather than being nitriding per se it could be the temperatures used by a particular mfr and the control of those temps that is the issue.

[LRRPF52]

Thanks for posting this - always useful to see and learn from failures. For a gun and cartridge supposedly considered for military adoption, it damned well better be able to keep up with that rate of fire. I've taken my Grendel 3-gun shooting, and there are video's from Grendel promoters showing full-auto fire. At 52,000 PSI rating, it should be able to take that all day.

For what it's worth, Wolf broke my Grendel extractor last week, on my PSA rig - at just 600 rounds in. 1st shot of the day. That rig has a carbine gas system. And carbine gas systems see a harder life in general. Combine that with Wolf steel, which is loaded on the high end of that (I think), and is steel, and things see a harder life. Still, it shouldn't have broke, and hopefully they will RMA that bolt for you.

In the meantime, I did find one place that has reasonably priced bolts and extractors. PSA supposedly will replace my broken extractor, but haven't yet. So I picked up a couple from Black Rifle Arms, which were under $20. They are selling bolts for $70. For all I know, it's junk; but heck - can't be any worse than the part that broke already.

From everything I've gathered, Wolf steel case doesn't generate much chamber pressure compared to most other factory loads, probably due to the undersized projectiles and 100gr weight.

The biggest contributors to broken bolts in any centerfire bottlenecked AR15 chambering are:

* Bad bolt metallurgy (8620 steel or poor controls on heat treating with any alloy)
* Excessive gas port size on CLGS or MLGS
* Shooting suppressed with a shorter gas system and longer plug dwell with a spec or larger port combined with a high efficiency can
* Receiver face not square
* Running high volume with hot loads

I've been shooting my 12" Grendel almost entirely suppressed with CLGS and 2.9oz buffers with a very high efficiency can, with one of the hottest factory loads out there (120gr Federal Match pushing 2400fps from a 12" Grendel) and I'm way more into a round count than the OP, so there is something wrong somewhere.

OP: What's your gas port diameter? MLGS 14.5" is normally pretty mild on the system.

[Growler]

OP - It probably wasn’t anything you did or didn’t do.

Keep in mind you can take a superbly designed part with specs that should ensure ideal process and still have failures.

Take the best manufacturer out there, ship them a few bolt stock forgings with some voids or inclusions hidden in the material before machining, get the heat treat temperature profile out limits due to a shift change, undetectable failures will occur. A proof load followed by mangaflux will not catch every defect. Few (maybe none?) of our bolts are 100% tested, but typically batch tested, auditing just a few parts per lot. The cost of 100% NDT (non-destructive test) is just too high.

Short of aerospace-level QC (which you will never see in any of these parts), failures will happen. If more of us had higher round counts, we?d see this more often.

* Don’t use another bolt from the same lot
* If another bolt fails with the same barrel, the barrel extension and carrier geometry may be suspect.
* Lapping the receiver with hand tools is hardly a precision job, but it’s low-hanging fruit. If it makes you happy, like collecting headspace gauge sets, go for it!

My 2c? Pick up a group buy bolt, do whatever else gives makes you comfortable, and get back to the range

[mdram]

Mdram, does that bolt have a coating of some kind?

not mine, i was just posting pic for the op

[Bronz]

not mine, i was just posting pic for the op

No coating

[Bronz]

Update:

Replaced the bolt with another bolt that was a spare. Same manf.

Took the rifle to the range today, shot another 60 rds which turned out to something like 72 rds. I kept stuffing rds. in three mags. Same Wolf ammo. This time just at 25 yards because I wanted the rifle to get hot. Started smoking on the third mag.
Took a video of it.

Rifle ran perfect.

[Bronz]

OP - It probably wasn’t anything you did or didn’t do.

Keep in mind you can take a superbly designed part with specs that should ensure ideal process and still have failures.

Take the best manufacturer out there, ship them a few bolt stock forgings with some voids or inclusions hidden in the material before machining, get the heat treat temperature profile out limits due to a shift change, undetectable failures will occur. A proof load followed by mangaflux will not catch every defect. Few (maybe none?) of our bolts are 100% tested, but typically batch tested, auditing just a few parts per lot. The cost of 100% NDT (non-destructive test) is just too high.

Short of aerospace-level QC (which you will never see in any of these parts), failures will happen. If more of us had higher round counts, we?d see this more often.

* Don’t use another bolt from the same lot
* If another bolt fails with the same barrel, the barrel extension and carrier geometry may be suspect.
* Lapping the receiver with hand tools is hardly a precision job, but it’s low-hanging fruit. If it makes you happy, like collecting headspace gauge sets, go for it!

My 2c? Pick up a group buy bolt, do whatever else gives makes you comfortable, and get back to the range

Well said brother.