WHat are you using to lube the necks with? If it's some kind of liqiud lube, you may not be getting them cleaned properly and some of the lube is left on the case. Just one possibility. Try a dry form of neck lube for the inside of the case mouths. GHD

Just shot 40 rounds threw my cz 527 (.223) today with my reloads that didnt split last time I shot them 6 months ago and they are from the same hundred round batch. they arent hight powered loads or anything either.
26.0 grains of RE 15 behind a 50 gr ballistic tip run .020 of the lands.
and i have used 26.5 grains with no pressure problems in the past. they are twice fired including today, wincheser cases. the splits are in the middle of the case neck, on 50% of the cases. Could this be from over-resizing? excess oil in the chamber?THANK YOU!

Neck splits are normaly from 1 of 2 causes, defective brass and case necks that are to hard. Is it a streight split, or is there a "T" shape on the end of it?

I had some first time shot factory winchester ammo 3006 brass split at the neck.

It was just bad brass or not anelled from the factory. I sent the box of fired brass in to winchester and they sent me a 50 piece bag of new brass.

its all straight splits, looks like theres little hydraulic dimples by the splits, can that cause them to split? might have gotten some wd-40 in the chamber.

I don`t think oil has anything to do with your neck splitting. The only experiance with neck splits I`ve had is in a 6.5 Swede that has a sloppy chamber. The neck is over worked when sizeing and splits from work hardening after a couple resizeings.
These cases aren`t reformed from another caliber or fired in a rifle other then the one you are loading them for? It sounds like a hard / poorly annealed brass problem.

I would say a case of defective brass. Back in the 70's I had a a bunch of .357 mag reminton brass spilt on the first fireing. Normally one could get 7 to 12 reloads but not with this lot.

Nope as good as the factorys are they do make mistakes.

The two possibilities I see are: older brass that has hardened either from overworking in the reloading process or exposed to pollution or both. If the sizing die doesn't match the chamber well and sizes the brass too much before it expands, that can also accelerate the process. Put that together with a larger sized chamber particularly in the neck area and you'll split the necks.

If the brass is relatively new and there isn't overworking in the couple of reloads, then you have defective brass.

Measure the neck of a piece of fired brass from that chamber. Then remove the expander in your die and size it then measure the neck. If the difference is more than about .010" you've got a die and chamber that isn't mated well. That will accelerate neck splits. The die manufacturer might be able to help a bit by polishing out the neck. Call them with the measurements and state your problem.

If that isn't the problem, it's the brass which might require annealing. That would certainly cure the problem.

I suspect the cases need to be annealed - if they are military brass, I think that they may be too brittle in the neck/shoulder region.