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New twist...separated NECKS!

fa18hooker

Silver $$ Contributor
Well, I was putting my new 6BRX AR-15 through its paces at 600 yards, when the bolt wouldn't close. Couldn't see any obstructions, but no matter what I tried, the round wouldn't chamber. Finally looked at my brass pile, and found the below pieces of brass...never seen that before!
 

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This looks like brass was neck turned as 6MMBR then fireformed to BRX or similar problem. The crack line is right where the old BR neck shoulder junction used to be. Was it new brass?
 
This was Norma brass, neck turned, fireformed, and shot a second time. Donuts to dollars it was my fault, turning the neck too low. I have yet to retrieve the neck from the chamber. The load was 31 grains of Varget behind a Sierra 107. The good news is the load hammered at 600 yards. It was only showing 2775 across the chrony (gas gun), which leaves some room...I'm going to try 32 grains next, and then maybe try Reloader 17 to try to get some more velocity.
 
Yep - When neck turning before fireforming you can only go down to above the new neck/shoulder junction and not below - a costly reminder. The blowing forward of the shoulder stretches the brass making thin even thinner - the further down you go the more it is stretched. Wildcats formed from blowing shoulder forward can have this issue and especially troublesome if necks are turned. Annealing helps to stretch without cracking. Esp in the AR platform where FL sizing and shoulder bumping required on every shot. the constant resize/fireform of shell will find the weakest lines.

To try and save this brass (expensive Norma so might want to try hard) you might need to anneal down to shoulder and minimum resize? But probably too late sorry. If it happened once I will bet it will happen again esp as you start to push the load. You can even see the line in the middle photo on shell on right.
Cam
 
I should add - I saw this years ago on 22K hornet and one brand of shells was quite thin (not neck turned though) and brittle. A lot of separation either at case head or very occasionally on shoulder.
 
The problem lies in the cutter of your neck turner.

You need to take a file or dremel and knock off the far sharp edge of the cutter where it's cutting a groove in the shoulder.

You can see the ring on the intact case, right where it separated on the other one.
 
I had this happen on a Lapua 6BR case fire formed to a 6 Dasher, after about 5 reloadings. I do not neck turn (0.272" chamber neck ID), so there was nothing done to the brass except the false shoulder and fire form routine. I have also had 3 other cases develop a circumferential crack right in the middle of the shoulder, but not separate. No ideas why, just the way it goes with a wildcat, I guess.

Cheers,
 
Luckily I didn't prep a whole bunch of brass! Here are the pics of the neck that I just fished out of the barrel...didn't think the shear was mid-shoulder, but sure enough. I'll fireform and THEN neck turn. Better yet, I'm going to see how it does with no-turn. The bolt gun cut with the same reamer seems to need it, so I assumed this does as well, but measuring brass indicates it doesn't. We'll see sometime this week...
 

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techshooter said:
I had this happen on a Lapua 6BR case fire formed to a 6 Dasher, after about 5 reloadings. I do not neck turn (0.272" chamber neck ID), so there was nothing done to the brass except the false shoulder and fire form routine. I have also had 3 other cases develop a circumferential crack right in the middle of the shoulder, but not separate. No ideas why, just the way it goes with a wildcat, I guess.

Cheers,
The problem is actually in fireforming these wildcats (turned or unturned) we blow the shoulder forward. What this means is the neck brass with typically12-13 thou neck wall thickness for total outside diameter of around .267-.269 will be stretched out. If you push the neck out all the way to shoulder or around half an inch - nearly doubling in diameter will double the circumference and halve the thickness of the brass leaving it too thin to function properly. In most cases we don't take it that far but then the next problem occurs----

In FA18Hookers case this is exagerated as he turned the brass down to the point where the shell is expanded the most. The line you see is where the thinnest shell is expanded the most and will be a stress point. Like most things brass will move at the point of least resistance so when it keeps getting fireformed and resized it will move most where it is thinnest - allah cracking.

Techshooter - you are seeing the same thing just without the neck turning and It is also what I saw in my K hornet. The shell is still the thinnest at the point of the shoulder where it is expanded the most. Fireing and brass creep just thins it more. At least with the dasher and steep shoulder angle there is less creep. the BRX suffers a little more.
 

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