Plastic Coating and Bullet Size

[Dimner]

Gas didn't crack the carrier, the severely deformed extractor pushed outward from the bolt to the inside of the carrier and that's where the carrier cracked.

 

[Ian]

So from the upper you can save the adjustable gas block, the gas tube, and the ejection port cover assembly. The rest is no bueno. Popper is right, check the lower and see if another upper will pin onto it, they're awfully thin in the right hand magwell area.

 

[Dimner]

lower is just fine. I have already assembled and disassembled the upper to the lower a few times in trying to figure out how to get the carrier out of there. already threw out the parts that I dont need. And yeah, I saved the upper components and just threw it out stripped.

 

[popper]

But the crack is on the other side. I understand what you are saying, it is possible. Extractor is not a full lug and can get pushed far enough. I suspect used brass failure. Had it happen in 40sw, head ejected but case stayed in chamber. The edge of yours looked the same as the 40SW case end (known bad FC brass). My BO overload was 2 gr too much H110. First one ejected OL, but bulged. Second was stuck tight - no damage to extractor. Rim was torn when wrenched out. Per GRT that was a 54k psi load.
223, 75gr nosler 16gr B.D. = 64k psi.

 

[Dimner]

I will take some more indepth pics and see if I can give a better idea of what is going on. Also, I'll run the actual QL numbers.

btw these types of exercises fascinate me. I enjoy a good autopsy. (of mechanical means)

 

[popper]

I would definitely get a 0.2 dia. rod to check load, Double charge is 75% but still can't see in that hole. Most real case head separations are due to reuse and excessive head space.
Anyway, you are learning. Tough it costs so much but better than body part hurts.

 

[Dimner]

So here are some additional photos. I now think the carrier crack happened as I used prying force to back out the carrier and bolt when everything was stuck together. Popper is right, the crack is on the side of the carrier opposite from the extractor.

I think what we are seeing is: "a little of column A, a little of column B"

the brass failed at high pressure, but not a crazy amount of pressure. My quick load calculations show a 72k psi chamber pressure for a double load. So while it's not a crazy amount over pressure, it still is too much. Too much for the brass. I probably have reused this brass 3-4 times for moderate pressure loads in the 30k-40kpsi chamber pressure range.

 

[fiver]

too much for the case for sure.

 

[CWLONGSHOT]

Defineate evidence of case "issues" shown in the pics. But you also see the deep indentation of the extractor and NO "sweep". Thats tell tail timing thing of a large excessive pressure spike. You can also see how the brass flowed around the bolt itself!

Brass plays such a small place in retaining pressure. About only places it does is where this blow out occured.In unsupported areas of a hand gun and the web areas in many rifles. IMHO NOT AT ALL telling me of brass "failure" it is a atypical over pressure signature.

I have seen more then a few quite similar blow outs. From various calibers.
CW

 

[popper]

Ugly~ Looks like the cam groove is beat up also. No ejector swipe as the case is stuck in the chamber pretty immediately. LC is tougher than most, 55K psi limit is test limit, not failure limit. My block doesn't turn the gas off completely.

 

[Ian]

the brass failed at high pressure, but not a crazy amount of pressure.

Really?

Let's take a little closer look. Your primer pocket is double the volume, the case head mushroomed, brass flowed fully into the extractor and ejector recesses, split your BCG, and the escaping pressure from the totally destroyed case head spread your upper. Hard case-head brass begins to flow somewhere around 65-70K psi.

The AR is a strong action and will contain "normal" case head separation or primer blowouts without much drama. What you have shows evidence of pressure well north of 100K psi. What Quickload cannot and does not predict is how powders designed for 12K psi can get STUPID with burn rate when subjected to high pressure. Blue Dot in particular is sketchy when taken out of its design parameters.

You got lucky this time. I'd abandon Blue Dot for future AR experiments.

 

[RBHarter]

72kpsi just vaporizes primers and opens pockets a little , at least in Mauser type actions with flat bolt faces . Been there in a Carcano because I didn't weigh a new to me bullet , and a 98 that decapitated a 22-250 due to heat . I'm glad the 6.8 wasn't in the AR ......

 

[Dimner]

Really?

Let's take a little closer look. Your primer pocket is double the volume, the case head mushroomed, brass flowed fully into the extractor and ejector recesses, split your BCG, and the escaping pressure from the totally destroyed case head spread your upper. Hard case-head brass begins to flow somewhere around 65-70K psi.

The AR is a strong action and will contain "normal" case head separation or primer blowouts without much drama. What you have shows evidence of pressure well north of 100K psi. What Quickload cannot and does not predict is how powders designed for 12K psi can get STUPID with burn rate when subjected to high pressure. Blue Dot in particular is sketchy when taken out of its design parameters.

You got lucky this time. I'd abandon Blue Dot for future AR experiments.

Well that sounds better. It did seem like alot of damage for something if it was only 70ish psi.


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