400,
Yep, that's what will happen when you use an expander which is too large for the projectile diameter.
In this case you were seating .408" bullets into a neck which has been expanded for .411" bullets.
When you're talking neck tension, 3 thou too large is a hell of a lot.
No wonder you experienced problems.

It wasn't the fault of the expander though, and unless the dies had been ordered to suit .408" bullets, but were supplied with an expander for .411", nor was it the fault of the die maker.

Don't fear the expander.
It is just part of a tool (the die) which needs to be correct in its sizing diameter to be able to do its job properly.
It is there for a reason.