The diameter ( .3575" to 358") has to be large enough for the 38 special. A fatter bullet may be sized smaller, if needed, for the 9mm. Lee lists a 38 bullet that they say, will work in the 9mm.
http://leeprecision.com/bullet-casti...double-cavity/ Wait for more replys as i dont load 9mm. LEE MOULD 358-125-RF SIX CAVITY may be the best for both.