New AR splitting case necks...

[DarkRose]

It is the brass.

Where did you get them as "once fired"?

I didn't get them anywhere, dad got the brass and did the reloading, I'm just helping with troubleshooting since I built the rifle.

I'd hazard a guess it was mostly Atterbury range pickup brass back when he was in the Guard, guessing by the dates of headstamps. (That would be the military ranges, not the civilian range). He got out in 78. Back then there was a lot more leeway about keeping range brass and such, from my understanding.

 

[DarkRose]

I fail to see what effect magazines or feed ramps would have on this issue.
For whatever reason, the brass is too brittle.
Since the fired brass chambered in a .223 bolt gun, I doubt that the neck area in the AR is so large that it's allowing the brass to expand to the point of failure.
It would be interesting to know what the outside diameter of the fired cases are. They should be around .250.

O.D. of the fired, NON-split cases is .254-.255", measured at 4 points around, Mitutoyo dial calipers.

Is that extra .004-.005" enough to cause the splits?

 

[Litlratt]

No