I have just finished shooting a few hundred .243 reloads that were loaded in 1999. No corrosion.
I also have a box of factory .260 Remington cartridges from the same time frame. Fully corroded cases around the necks. Same storage conditions as my .243 reloads (unheated garage).
Only differerence I can tell is my .243 reloads were moly coated. There may be your answer.
