So what caused my cases to split?

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At my last High Power Rifle competition I was shooting my AR-15 (extremely poorly I might ad) I finished my last stage of slow fire and put my rifle away. When I went back to my shooting mat I wondered how a 9mm case got on it. I picked it up and noticed it was what was left on a 5.56 case. Now I know I have a broken shell in my rifle, or assume it was mine. I was on my 3rd loading of Lake City 04 brass.

I got home and sure as shit I have a half a case in my rifle and no broken shell extractor.

Since I've never had this happen before, what caused this? It looks like something got progressively worse.

Now this is my competition rifle I've only shot starting last year. Probably only 500-600 rounds through it. It was also cleaned the day before the match.

As far as loads go, I use 69g Sierra Match King bullets and 24.5g of AA2520


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Check the headspace.

When I have seen split cases on new ammunition / newly reloaded, it is due to the headspace being too long. Have you checked the headspace? I would before i take that rifle out again.
 
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You may be over-sizing the brass. How far back are you pushing the shoulders?
 
Yes, Headspacing could do it, But the thing to remember "Gas Guns" are hell on brass. Start checking your other brass for a stretch ring.

This I've never done. I've been shooting high power for about 12 years with the same load and never had a problem or issue. I'm going to scribe all the cases from this last match. I've already set them aside.

Lcstyle I trim my cases every time I shoot them. And yes I also lube every case when resizing, but never have I annealed them. None of the necks split either.
 
Out of curiosity, what were the perceived symptoms when the case fully separated? Did you notice something different or is it just dumb luck you didn't try to put another round in the chamber?
 
I've had cases split in matches and the bullet still found the 10 ring. I didn't notice until I tried to chamber another round.
 
the wiki article explains the process quite succinctly:

When a firearm has more headspace than the cartridge design anticipated, the closing of the bolt or impact of the firing pin may move the cartridge forward to leave space between the chamber face of the action and the base of the cartridge. Pressure of burning powder gasses expands the thinner forward walls of the cartridge case to firmly grip against the sides of the chamber preventing rearward motion. The thicker base of the cartridge case (or sometimes the primer) may then be forced back into available space at the rear of the chamber, causing the case to stretch. Where rearward movement exceeds cartridge design, the primer may rupture or the base of the cartridge case may begin to separate from the forward walls.
 
What type of dies are you using full length or short base? Short base work the brass harder

I've also seen improperly setup dies bump the shoulder back over .0020

Verify with a case gauge but I only bump the shoulder back about .003 to .005 with a FL redding die.

Your also on the hot side of the load at approximately 53k psi possibly higher depending on primer and OAL among other factors. (I checked @ 2.260) with a rough check in quickload so too much headspace along with a slightly compressed hot load can spit a case quickly, have you looked for overpressure signs?
 
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Bend a paperclip into a small L at the end, and insert it into your fired brass and run it from the bottom to up along the side and see if you feel a groove inside the brass.
 
Primers to me can be a tough read my AR flattens CCI 400 at any load with correct sizing but not #450 magnum primers which I usually use. I have a 9mm sig that does that too even flattens factory ammo

Though I will agree it's almost certainly overpressure, the last case almost looks pierced
 
They all look a little over pressure. cratering around the firing pin?

It's not pressure. On a bottleneck cartridge that headspaces on the shoulder, excessive headspace always results in a flattened primer. Always.

OP - Do you use a case gage, precision Mic, or headspace comparator when you set up your sizing die?

As Jason pointed out, you're probably pushing the shoulder back too far when you're sizing the cases. In your case, the excessive headspace isn't being caused by the chamber being too long, it's that the headspace length of your cartridges is too short.
 
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