If the cracks are forming in the neck, neck sizing won't help if you have a larger chamber. Some brass is more brittle than others and I suspect military brass is designed for single use unlike Lapua or the other fancy stuff. Even so, the firing process can put a lot of strain on the brass as well as resizing (both activities work harden the brass). Eventually, the stress is too much for it and it cracks (unless it is annealed which softens the brass).