I happen to have been surfing 300 whisper info when I ran across your question. The useful case capacity of the 300's parent cartridge(221 Fireball) is 1.43 cc of water. To get the capacity in grains you must multiply this by the VMD (volume of one grain of powder) for the particular powder you are going to use so there is no definite answer to your question. You will have to measure the amount yourself or Lees Reloading Manual has that type of info in it. Hope this helps out.
Now if I could just find a cheap 300/221 ar barrel I would have a nice upper for myself and I could give you all kinds of info.