Hotlead,
I'm new to the 300-221/Whisper/Fireball caliber myself. In my reading I have found that, 300-221 chambers are the tightest with whisper next and fireball the largest. I think the differences in the chambers is pretty small and is just a way to keep from paying royalties on JD Jones patent. Are you planning on using necked up 221 brass or cut down 223 brass? I don't know what JD Jones was using. I think the only possible problem might be neck diameter. That can be corrected by reaming the necks. My redding dies using cut down 5.56 brass have a max case neck of .335 which I believe is right at the max for a 300-221 dimensions. This was without any reaming so I would think your dies and brass from JD Jones should work fine. Wow that was wordy! I'm sure one of the more knowlegeable folks will be along to correct me or verify soon.
