split case necks.

Figure out how much the neck grew during firing and you will have your answer. If Kimber rifles are up to the same standard as Kimber pistols, I bet the chamber is cut wrong. Oversized, burr, spiral, etc. I guess the all important question is what does it do with factory loads.
 
This only happens with once fired brass right? no factory ammo has this issue?
If so its a Brass hardness,batch issue. Try annealing or throw it away and try another batch or brand. Sizing hardens brass. It may be extremely thin brass from the factory and sizing is all it can take.
 
Take the decapping pin out of your sizing die, and inspect the inside of the die. I had a similar issue a few years ago with a .221 Rem Fireball sizing die. There was a bur inside the area of the neck. I could have sent the die back to the company that made it, and they would have fixed or replaced it. I did not want to wait so I went to a local machine shop, and a machinist charged me $15 to clean up, and polish the inside of the die body, and to true the expander. The result was it stopped doing just what I see on that brass.

I think they call it galling inside of the sizing die.
 
RC20, I was thinking that it looks like what you're talking about. Maybe from using to much lube when sizing and causing a slight dent or inclusion in the brass. Then when fired caused the separation that we're seeing in the picture.
 
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