back in the mid 70's i sent a mini 14 down to a guy in texas that rebarreled them into shooters. I had him chamber it in 6x45. the rifle was great shooting 3/4" groups, and then got put back into the back oif the rack for several years. When i took it out again about 1 in 7 cases would separate in the middle of the case. Checking the headspace i find that it is fine. I thought that maybe the die had pushed the shoulders back to far, but that checked ok too. Next i took some 223 brass (everything is gi brass mixed) and carefully just ran the expander nipple through and loaded it again (23 gr h335 and 70 grain bullet)this time i got through about 25 rounds before it started happening again. in looking at the fired cases most looked perfectly fine, but every so often i could see a ring where the separation would occur. anybody got any ideas????

When I got case separations involving 5.56 in M16s having normal headspace, the cause was most always a very dirty chamber.

You mentioned the rifle sat in a rack for several years. Can you get a good look at the chamber walls?

well thats something i never thought ofl I'll see if my borescope will work

This may not be you're problem, but old brass that has been laying idle for a long time can behave exactly the way you are seeing. I think there is some kind of oxidation taking place inside the cases that do this.

Brass quality is my guess too

Hodgdon data thinks your below the starting powder charge, and so do I. But may be ok with GI brass ???

I had a dillon shell plate that gave different shoulder set back at each station. This caused separations in some, not all 223 for me.

what have you come up with? was it LC brass?

So the brass is the same vintage as the rebarrel? Might be stress corrosion cracking. Was there any ammonia present at any point in its storage life?

still working on it the brass is mixed gi but mostly lc most of the stuff is old brass, but the last batch was newer. next thing to try is to get some new brass probably have to use commercial brass (never done that before)

Have you taken any apart to see if they are starting to show a separation? I had a batch of .222 that looked good but they were old reloads. I had two that separated and descided to scrap the batch and found they were actually very thin just above the pressure ring. I cant say how many tines I had reloaded them maybe 10 or 12 times. I split a couple long ways and you can could see it clearly. I know its not military.

old brass could be of issue and a very sticky chamber --- think oil turned to varnish in the chamber?