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I recently purchased a 7-30 JDJ barrel from a guy and got 150 fireformed brass with the barrel. I took it out last weekend and fired 20 rounds through it. These cases are all 30-30 cases fire formed and sized. All but 2 cases cracked on the necks vertically towards the rifling. The necks were annealed prior to loading and there were no overpressure signs. Does anyone know what is going on with this? Is it common for this round or is there another issue.
Sounds like you should have annealed them first.
With having annealed them prior to loading, I'd say they must have been old and used hard. Even with your annealing, they don't last forever. The chamber might too generous. Wonder if you could get a chamber cast to inspect the dimensions? With fairly new or low shot count brass, I've never had that happen.
The cases could have just plumb worn out, they do not last forever. You don't need to pour a chamber cast, just measure the fired case neck diameter vs. the loaded rounds.
If you can, buy some new or once fired 30/30 brass and size them down and fireform them yourself. Definitely anneal them. I once had a standard 7-30 waters T/C barrel and never had any problem with my owned formed brass. It was a deadly accurate cartridge.
The JDJ improved should not pose any problem, probably just old brittle brass that the fellow gave you.
I also have a 8mm JDJ barrel that is made from .444 Marlin brass, and it is no problem whatsoever making and fireforming brass.
I've had split necks and shoulders when fire forming used brass even after annealing it but have not had this happen once formed. You should check the dimensions before and after.
What were the loads? I size my 7-30 cases and then fire form with 7-10 gr Unique. They shoot well at 50 yards. I then take those "Fire formed" cases and load them with full loads. Since I am using older fired brass I expect to lose 3-4 cases per hundred. Is lit possible that your loads were sized for 7-30 but not fire formed? If so, I can see the amount of loss especially with older brass. I would toss the two that didn't crack as they probably soon will. Time to start over.