Not a great way to begin a thread on load development but....

[Fast14riot]

That looks more like a clearance cut for the M16 extractor.

 

[M60A3TTS]

Wasn't Christensen the ones, who had all the chambering and accuracy problems back a years or so ago?? 3.5 on Google, and some horrific reviews.

 

[farmerjohn]

My thoughts-
1.Defective brass failed?
The hot gas went thru the M16 extractor cut.
The vented gas split the receiver?

OR

2. Did the receiver fail first, because of improper heat treating?
Then the case head moved off the bolt face and case head ruptured?

I think #1

Best part, no one was injured.

I went back and read up on Ackley and the early Springfields with improper heat treatment and I'm going with your number 2..... John

 

[farmerjohn]

I haven't read through the entire thread and probably have not seen all of the pictures but, as of page three, this is my guess:
It looks to me like the action was compromised during heat treat, then compromised even more by torque on an inside wrench. The incipient lengthwise cracking was primarily at the top of the right-hand raceway. I would expect the failure began with the second last shot and there was actually significant head clearance on the last shot.
The end result is kind of a culmination of events, a perfect storm, of sorts. Receiver not right from the start, damaged during barrel installation, damaged further by the firing of proof load, and finished off by normal used. A freak accident, in a way. I have seen some modern rifles and some not so modern, which were destroyed by severe overloads, and other which were destroyed by damage by the "gunsmith". These were rifles which had barrel threads which were too tight, or which had been damaged by the use of an inside wrench. They looked just like yours. They came apart at the receiver ring, split lengthwise. The cracks showed signs of being penetrated by solvent. In other words, they had been there for a while. The guns came apart suddenly, with what should have been a normal load.
Now, I have not seen your rifle personally. There also a chance I could be wrong (happens all the time!), but my guess is the action was flawed. WH

I think you nailed it , those thread's are straight and sharp on the barrel and the receiver, not pulled. In my little pea brain I can see the threaded end of the receiver turning loose outwardly suddenly and the barrel moving with out pulling the threads. Just my thinking... John

 

[Kracken McGraw]

Thanks to the OP for sharing his load practices and for the information about no pistol powder being around. Sure sounds like he was doing a good job on his part on being safety conscious with his loading.

 

[ELR LVR]

Not sure. I'll check tomorrow if I can inspect the bolt all the way around. I like the idea of a vented bolt going forward.

Yes, this is a safety measure another gunsmith showed me years ago
So I vent most all my bolts now
it was mainly to help vent with a pierced primer
but I would bet it would have helped a lot in this case as well
---
And I machine in 2 vents, not just one

 

[Will Henry]

Having said what I did, I still have to go along with Dave as well. For brass to deform like that, there has to be some pressure involved. WH

 

[DShortt]

Having said what I did, I still have to go along with Dave as well. For brass to deform like that, there has to be some pressure involved. WH

Oh, the OP was definitely on the upper end of the scale.
But.
Unless there was an excessively heavy charge or an excessively light charge if you consider the SEE effect or the receiver was compromised when made, it shouldn't have split like that.

Details we'll never know.

I just don't get how high pressure would result in so much apparent bolt thrust without splitting the barrel as well.

Again, details we'll never know.

Lube up the case exterior with something, even a powdered something......

That I can see.

 

[243winxb]

So I vent most all my bolts now
it was mainly to help vent with a pierced primer

Christensen arms Mesa bolt. Link

 

[jackieschmidt]

To split an action like that, there had to be a tremendous over pressure or a mechanical failure.

Going back over 70 years, POAckley did severe over pressure test on various military actions to see, in simple terms, what took to destroy them.

On quite a few of the actions, the receiver ring did split with the bolt still in place. He estimated that it took over 100,000 psi to do this, and only if the case ruptured.

There is a possibility that the action had a flaw or possibly developed a crack in a critical spot.

Or possibly they missed the heat treatment. Many of these are made from 416 stainless, which is chosen for it’s machinability, not overall strength.

 

[Kracken McGraw]

Two more safety checks that might need attention is to look in the case before loading for anything that shouldn’t be in there with a clear view of your new primer, and the other thing is after you empty your powder funnel each time tilt the thing up and make sure there’s no powder bridged in the tube.

 

[Doom]

I just don't get how high pressure would result in so much apparent bolt thrust without splitting the barrel as well.

No part of the receiver/action is designed to contain chamber pressure by itself. For all practical purposes the only pressure containing portion is the threaded portion that accepts the tenon. Chamber pressure is transmitted from the gasses in the case to the thin wall of the case which expands to the tenon inner wall. The tenon and treaded portion of the receiver then expand based on the stress/strain relationship. Essentially the action replaces the material removed from the barrel in machining the tenon.

If the lugs had failed the bolt would've been ejected. The lug point of failure would be at the wall of the action or bolt.

The action was blown apart. The only portion of the action subject to chamber pressure is the threaded portion. The area between the lugs and the end of the tenon is not design for chamber pressure but can become pressurized if the case fails. Even if the action were cracked it would not have blown apart unless the tenon/action connection failed which would be evident in the threaded areas of the action and tenon. I see no evidence of that in the photos.

As far as the photo evidence is concerned it looks like the case failed (for unknown reasons) and pressurized the area between the lugs and tenon which overpressure the action and blew it apart. Assuming the vent holes are open then this would seem to be an extreme overpressure event. It is possible that the case was the issue but it's unlikely to be able prove that.

The OP mentions Chargemaster and one potential source of overpressure could be bumping the pan on the dispensing tube when moving the pan from the platen.

It's time to move on.