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JD Jones copyrighted the name whisper so the same round by any other source has a different name hence 30-221, 300-221, 30 or 300 fireball etc. They are essentially the same thing, use the same load data and bullets and have the same ballistics.
I have no idea on the AR stuff so I can't answer those questions.
Barrel length depends on your intended purpose. Longer barrels can give you more velocity if you load for them so if you want high vel hunting ammo to do well thats a consideration. If you only want subsonics than short barrels are fine. You can develop 1050fps from a pretty short barrel reliably but getting 2500fps from a 125gr slug may require a longer tube so the choice is yours.
I use a variety of .223 brass to make my 300-221 brass and I don't normally anneal at all. The working of the brass is not very extensive when loading subsonic and doesn't seem to harden it much. With a tight chamber the brass doesn't get worked much either so it is unlikely to crack soon. I've got cases fired 15 times with no cracks. Annealing will change the grip the neck has on the bullet so that can affect accuracy. Now if you are loading supersonic than you may need to anneal occasionally since the brass will flow or work more.
Hopefully some others will chime in here as we all have different angles and experience....
Frank
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