I'm currently in load development with a 300 Blackout 10" AR pistol and Ops Inc 3rd Model suppressor. My powder is IMR 4227 and I've been shooting Hornady Interlock 220 gr round nose and 180 grain Spire Point bullets. Neither bullet is designed for subsonic purposes but they are what I had on hand. I'm loading 8.7 to 10 grains of powder and getting velocities from 887 to 1029 with excellent accuracy out to 100 yards. Yesterday I got a delivery of Hornady SUB-X 190gr that ARE designed for subsonic use and I'll be shooting them tomorrow or Monday. I already loaded a few rounds with 9, 9.5 and 10.4 (max load) grains of 4227 and should get great results.
