Picture of a catastrophic bolt carrier group failure?

[Chills1994]

Hi all,

I pulled this pic off another forum. It happened during a training class.

Has this ever happened to you?

Are there any warning signs that this about to happen?

Are there bolts or bolt carrier groups that are known to do this?

Or are known for their superior BCG's?

Is the upper going to be mangled up on the inside (is it still useable)?

TIA!

[busyhawk]

Chills1994,

Never and hope to ever see this kind of break...can't imagine what would make this happen to a bolt. I do see that the bolt is broken in a weak point (Should see cracks if cleaning all the time). Our Army guns always break a locking lug and most of the time will not know of the problem until we take it appart...meaning it will still function 85% of the time. Don't see why the upper/the rest of the gun shouldn't be fine. Would need more information. I can't see why the bolt carrier should not be fine either.

Busyhawk

[GentlemanJim]

Years ago when I was building lots of AR15s I encountered this problem of bolts breaking in the cam pin area.

It turned out to be a batch of bolts that were incorrectly hardened...and were too brittle.

But other things may also result in this failure.

When I was breaking them...no damage to the reciever...But I would check the barrel extention VERY carefully.

Jim

[gringop]

Ive had it happen in a match. The bolt was from Model1sales. I have since replaced it with a LMT MPI bolt. Sometimes it's worth spending the extra bucks.

Gringop

[p99shooter]

OT but: Anyone want to guess what state that rifle is from?

[Flatland Shooter]

OT but: Anyone want to guess what state that rifle is from?

A state with high humidity and very little sand? I've never seen that much oil come out of an AR before!

Bill

[p99shooter]

OT but: Anyone want to guess what state that rifle is from?

A state with high humidity and very little sand? I've never seen that much oil come out of an AR before!

Bill

Hah! +1 on the sand, but negative on the humidity.

[Sestock]

I had a bolt blow like that on a first generation Professional Ordnance Carbon 15. I think the first run of bolts were lightened to the extreme. The replaced it with a bolt with a little more metal on it and it worked great.

BTW, I was shooting Remington UMC Yellow box ammo at the time.

[jbullgpd]

Stock bolt or aftermarket? Never seen it in a reputable manufacturer....

[kurtm]

Yes I have seen a few of them over the years. A Bushmaster, a couple of early Armalites, and a DPMS long ago. Usually they only crack one side of the bolt and then it all jams up inside the bolt carrier. At least that one will be easy to get out! A couple of things came to mind, could be the problem or not I don't know, but these were the thoughts I had. The bolt cam pins was very worn in both Armalites, this may have caused the bolt to beat around on the pin more...just a guess. ON the DPMS the bolt carrier was hitting the extention so hard it had actually peened it a bit over the years and the extentions are harder that the hinges on the gates of hell. The guy used to drop the bolt on an empty chamber A LOT! while dry practicing for Hi Power...maybe impact related?? I don't know. The Bushmaster was a mistery and we chalked it up to bad heat treating of the bolt, it only had 1500 rounds through it, and the guy didn't want to wait on Bushy to warranty, so we pryed it apart and with a couple of hours work and a new bolt it was back to shooting. The nice thing is this isn't a KABOOM failure and no one was hurt! KURTM

[MyBoyElroy]

Hydrogen embrittlement. Bad heat-treat. Expensive bolt from big name source.

Went to Colt MP C bolts and never another problem.

[blkbrd]

All metal parts will fatigue and fail.

Its a matter of preventing it for as long as possible with proper material selection, machining practices, de burring, heat treatment and stress relief. Then proper use.

Bolts and carriers are done in huge batches, was yours done on Friday at the edge of the bar or the basket in the oven???

[wsimpso1]

Hmmm, If'n I was a betting man, I would bet that the subject gun had a 16" barrel and a shorty gas system. In rifle length and mid length gas systems, the action gets a nice reliable push with the chamber pressure bled off and the cartridge case loose from the chamber. But in the shorties with a 16" barrel, the system gets its gas earlier, the gas gets to the carrier sooner, and chamber pressure is not bled down as much. The result is the shorty action is run harder. Then, when one gives difficulty extracting, some folks drill out the gas port, really beating the bejuses out of the bolt and cam pin. Add in having to open the gun from stuck cartridges, and there is more abuse. Better parts might help, but I suspect the gun is driving the bolt pretty hard.

Hydrogen embrittlement (MyBoyElroy's picture and post) can generally only happen if the part has been plated, which the photo part appears to be. The silly thing about hydrogen embrittlement is that a one hour bake at 350 F right after plating drives off the atomic hydrogen and positively prevents the cracking. The shop that provided that bolt (or their plating shop) has lousy QC and should be avoided like the plague. Sounds like you already did... Good deal on warning the rest of us about them.

Billski

[MedicineMan]

I had a similar failure on a ZM LR-300 11.5 in barrel, the barrel and bolt had at least 5000 rounds down range and let go at the very end of a carbine class. The bolt fractured at the Cam pin hole. In retrospect there were some early warning signs that I ignored and instead shot some more lube to it. I think the bolt had cracked on 1 side before it finally let go all the way. No other damage was done to self or gun.

[Loves2Shoot]

I had it happen on a fairly new Olympic Arms rifle once.

[Larry White]

Anybody take a good look at the case shown with that bolt in myboyelroys pic? Pressure a little high? Dont guess that could have had any thing to add?--------Larry

[Loves2Shoot]

Do you mean it either fired out of battery or he shoots a hybrid round?

[Larry White]

I mean is was overpressure enough to seperate the neck from the case body and has the begining of a case head seperation and I really cant tell but it looks like it tried to split down the side of the case. That was a overpressured round. Maybe not the fault of the bolt.-------------Larry

[wsimpso1]

The case in MyBoyElroy's post looks like they look when the bolt lugs break. The chamber is still under a bunch of pressure while this is going on:

The bolt sets back (sometimes with vigor!);

The neck, which is stuck to the chamber wall stays put;

The case fails at the neck/ shoulder junction or at the body shoulder junction;

The head/body area is stretched and looks like an incipient head separation;

The headspace length of the case is way long;

You can not even get that case into a chamber check gauge because the body is expanded.

Now, don't get me wrong, the round could have been high pressure, all that i am saying is that even a normal round in the gun would look like that when all of the bolt lugs break off like that.

I have seen films and the aftermath of a number of destructive firings of rifles. All kinds of things get revealed when firing with bullets stuck in the bore. One competitor's bolt rifle broke the front ring, retained the bolt but the barrel went downrange, with the case held by the extractor, and then as the barrel cleared the case, it ejected the case. Piezo pressure gauge routinely showed way over 100,000 psi on this type of firing. The case was significantly bigger in diameter and longer at the shoulder than when it went in, but it did not split.

The other reason that I do not believe that high pressure ammo did this to that bolt exists. High Power competitors have a subset of shooters who are infamous for running (in 20" Barreled Service Rifles and in longer barreled Space Guns) rounds that are well over standard pressure in what the rest of us feel are misguided attempts to get more velocity. In guns with long gas systems, they run that level just fine, rebarrel at 4000-7000 rounds, and just keep on reusing a known bolt and carrier. Yeah, they replace extractors and cam pins with the barrel (and dig primers out of their triggers on hot days), but broken bolts are not commonplace. It is in Shorties that folks see cracked bolts, cracked cam pins, etc. And if the bolt was plated (that one was) and the shop skipped the bake cycle, it would be very brittle and fail quickly.

Billski

[MyBoyElroy]

Anybody take a good look at the case shown with that bolt in myboyelroys pic? Pressure a little high? Dont guess that could have had any thing to add?--------Larry

Genius.

Wisdom like this is why I don't frequent this site.

The round in question is factory WWB. Less than ten rounds out of a new rifle to fragment the bolt.

Bolt went back to vendor. Vendor did not manufacture or finish the bolt but sold it as part of an assembly. Vendor did failure analysis and reported hydrogen embrittlment as the proximate cause.

Got two new assemblies in return. They are sitting in my spares box for use on somebody else's rifle.

Try again.

[Larry White]

Anybody take a good look at the case shown with that bolt in myboyelroys pic? Pressure a little high? Dont guess that could have had any thing to add?--------Larry

Genius.

Wisdom like this is why I don't frequent this site.

The round in question is factory WWB. Less than ten rounds out of a new rifle to fragment the bolt.

Bolt went back to vendor. Vendor did not manufacture or finish the bolt but sold it as part of an assembly. Vendor did failure analysis and reported hydrogen embrittlment as the proximate cause.

Got two new assemblies in return. They are sitting in my spares box for use on somebody else's rifle.

Try again.

Excuse me for asking a question. Maybe you could have provided that info to start with. I remember you from when you were here before. Havent noticed you dont frequent this site anymore. Wonder if anyone missed you.---------Larry

[J-Ho]

I saw a model 1 sales bolt break in pretty much the same place. If I remember correctly, the owner said he was only a few hundred rounds into the gun.

[wsimpso1]

Hydrogen Embrittlement - This happens when steel is plated. Atomic hydrogen is evolved at the surface, and it wanders into the metal. The steel become weaker and more brittle, and will fail quickly if highly stressed. The silly thing about all of this is that the prevention has been known for about 100 years. The shop that does the chrome is supposed to bake the parts at 350 F for a couple hours to drive off the hydrogen. MyBoyElroy did mention that hydrogen embrittlement was the source...

We can also see short lived bolts if the heat treat shop quenched the parts, but tempered them wrong. Quenching makes the austenite (steel above its upper critical temperature, usually above 1350-1500 F) into primary martensite. This stuff is hard as glass, has huge stresses locked up internally, and is fragile like glass too. Tempering relieves the internal stresses, which increases the toughness in a big way. The higher the tempering temperature, the more hardness you give up inexchange for more toughness. Screw up and skip this step, or run it too cold or only for a short time, and the parts will quickly fail.

Billski