AA#5 is a bit too fast for the 300-221 Fireball*. A better choice of powder would be AA#9, H110, LilGun or W296. I have had the best results with W296.
I have posted some of the loads that I have worked up for the 300-221 Fireball here:
http://www.reloadersnest.com/frontpa...?CaliberID=135
Here are some results for H110
http://www.reloadersnest.com/unverif...ID=135&ID=3213
*Note: the 300-221 Fireball, 300 Whisper and 300 Blackout are all pretty much the same cartridge, with only slight variations in the neck dimension.
320pf