I tried this about two years ago. The speed was not what I had hoped for and the SD of velocity was pretty wide. Maybe someday they will produce a powder that will make the .25 WSM work better . . .

The 6.5 WSM is a real good WSM wildcat; and it you want a really screaming .25 caliber, the .257 STW is the ultimate. Complete with all the ear splitting noise and short barrel life that goes with such an incredibly over bore caliber.