Quote Originally Posted by MikB View Post
...Cases with a tight in/out bend at the shoulder like 303 tend to work-harden there in manufacture, and if the chamber is reamed deep - as it often is - the case stretches at the web in fire-forming rather than rolling the shoulder forward. Even neck-sizing only can't remove the risk of separation in that situation.
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I think a big part of the issues with a .303 is the case headspaces off the rim and with generous chambers the brass gets very thin near the head on only a few firings.

I have had separations with S&B brass 100% of the times I have reloaded them (on ever first reload). Admittedly I have stopped using them so might find one that lasts for a second shot and I don't anneal if that makes any difference to them.
However, I have reloaded the PPU brass 3 times and so far no separations. With several .303s to load for I full-length size each time, not just neck sizing, so giving them a good workout.