One of my absolute favorite cast bullets for 300 blackout in a silenced, subsonic AR is the Lyman 311679, also called the MXL bullet. NOE has just produced a version and I have a 5 cavity on the way. I can't wait to start dropping big piles of them to feed my hungry AR.
The 5-6 cavity pistol bullet molds have really spoiled me for guns (like the AR) that I shoot in large volumes. I'm really looking forward to dropping more than two at a time.