The cartridge was originally developed by JD Jones and is commonly known as the 300 Whisper.
The 300-221 cartridge is not available in the featherweight barrel profile.
This cartridge is formed by full length sizing 223 brass in a 300-221 die with expander removed and trim to 1.4"
Loading Data is available in many loading manuals as well as here.
Velocities in rifle barrels will exceed the pistol barrel data that is commonly available.
We recommend an adjustable gas tube with this round so you can use both ends of it's performance range.

Above is a good cartridge comparison picture and the 2nd group that I shot with my barrel last year.

Forming from 223 Brass: Left to Right are: 223 Parent Case After sizing in 300-221 sizer with expander stem removed Trimmed to 1.4"