Mustang, I sent an information request to Hornady and here is their reply. (I'm heading out this morning to try 5 each of 10.4, 10.6 and 10.8 grains of A1680 with the 220 grain bullet, and will let you know what happens.)
We would suggest you try A1680 with a start load of 10.4gr and a max of 15.3gr and a C.O.L. of 2.250". Start low and work your way up and you should find the most optimal load that will work for you.
From: Hornady Manufacturing, Inc [mailto:firstname.lastname@example.org]
Sent: Thursday, January 31, 2013 8:33 AM
Subject: "Technical Inquiries" Contact from mike innis
This is a response sent by mike innis using the feedback form on the website. The details of the message follow below:
410 Broad Street
I am attempting to work up loads for a 300 Whisper and have had difficulty developing a subsonic hand load that will reliably cycle the action on the AR. I friend gave me 5 of your factory loads to try, and they worked quite well! Is the powder you use in your factory, 208 grain AMAX ammunition for the Whisper a proprietary blend, or is it a commercially available powder?
Thanks very much for your help!