If I ever go back to loading for my late father's 300 Win mag I'm not going to shoot new brass as-is. It'll be expanded a bit and then sized to a false shoulder for zero "shoulder headspace" from the get go.
This probably won't stop the stretching IME. The problem is sloppy chambers and short cases. Brass has to flow from somewhere to fill the chamber and the area in front of the belt is where it comes from.
