Maybe this has something to do with it:
"Old" Winchester primers, at least small rifle primers, were nickel plated. The new ones are just brass. The old ones are slightly larger in diameter than the new ones.
For example, using .223 WW brass, Remington 7-1/2 (brass) develop loose pockets with about 5 reloads. Old Winchesters are good for at least 10 reloads, after which neck splitting starts to occur. This with 24 grains of Varget and 69 grain SMKs, a load that is not particularly hot.
