sabot in .300 whisper
 

would this be going backwards or would this have any use? http://www.reloadammo.com/sabot.htm

I've thought about this, I use the sabots for my 308, but it seems like you would be right back at a 221 fireball.

Interesting. Not sure what it would accomplish except maybe give you the ability to use a Whisper as a varmint gun as well? Should be able to get decent velocities...