Just sizing & loading a piece of brass can work harden it enough to crack necks, especially if it wasn't annealed properly at the factory. If you experience this pull some of the bullets and look for the blue/white, powdery "stuff" that indicates moisture corrosion. Powder will be clumped together & bullet bases will have visible signs of corrosion. Mice can cause the ammonia corrosion just by wizzing around the box the ammo is kept in! They have mean digestive systems for such little s.o. - um, critters (I hate mice!). A buddy of mine experienced this with some WW 7.62x39 factory ammo. Some of the pulled bullets looked they had been stored in a salt water aquarium! Exterior of carts & boxes looked perfect but they had absorbed moisture like soda crackers! Powder clumped, dead primers... it was a mess!
Keep it sealed up as best as possible & control the temps within reason and ammo will generally out last every one of us by a generation or more. I've shot WW1 .45 Auto (1917 & 1918) that functioned perfectly, even a cardboard box of 50 primed, but unloaded 45 brass from the same era (Rem UMC) that functioned perfectly! It's all in how it's stored... with maybe a bit of voodoo luck (good or bad) thrown in!
Cheers,
crkckr