At present I have only one .300 Whisper - an Encore SS 12" T/C Custom Shop barrel that the previous owner ordered in .30 Herrett. I sent it to Mike Bellm in Grants Pass OR, who reamed it out to .300 Whisp-R improved.
Since my barrel already had the rim cut for its original cartridge based on the .30-30 Winchester, I have the option of using either .221 Fireball or .357 Rem Maximum brass to make .300 Whisp-R improved.
I have been making them one at a time but it is slow. Mike Bellm recommends annealing the cases prior to resizing or they will split. Using .357 Rem Max brass does give a insignificant increase in case capacity but with sharper shoulders.
http://www.bellmtcs.com/store/index.php?cid=570
My dies are from CH4D and are marked .300 Bellm - Whisper.
http://www.ch4d.com/
Can I send .357 Rem Max brass and have it treated the same way as the other brass?