I can't speak to AR loads for 300 BLK, but my quietest load for my bolt rifle was using Unique powder with Hornady 190 Subx.
I spent a lot of time and components working up that load but realized that the "whop" of that 190 grain slapping pork will scatter pigs as much as a louder report from the rifle. Then mission change for my 300 BLK and I decided to use up the CFEBLK that I have on subsonic pig hunting loads. It's a little louder but actually groups more consistent.
Unique has been real hard to get as well.