Here's the RCBS die for necking 30 cal to 400, Angus. You might also be able to use a 40 cal pistol die expander. Never tried it, so not sure about that.
Lube the inside of the standard neck well, bottom case, and a single pass through the die gets you the case in the middle of this picture. The top case has formed the brass to a slight crush fit in the chamber. Chambering the case actually completes the forming process.
The best part is that you load the reformed brass with whatever load you need to. No need to have an intermediate fireforming step. These are actual hunting loads.
Full power loads create perfectly formed and headspaced brass.
Simple as 1, 2, 3.
Ted