this was quick: brass, bullets and die set

the 414 super magnum is 40,80mm long, CIP length for the 401 WSL is 38,10mm. I use the case hand trimmer and its good enough. rim diameter is 11,68mm max for the 401 so I turned it down on the lathe. the Lee 41 mag carbide die set will full lenght rezise this brass without problems
diameter on case mouth 10,91mm amnd base 11,00mm. CIP measurments for the 401 are 10,99 case mouth and 11,00mm base.
to good to be true

side by side: original 414 supermag and 401 WSL reworked

the case fits fine into the chamber, the hammer strikes with force