Have used Lehigh bullets very successfully and Outlaw State bullets also work well. 2 completely different approaches to the same problem of getting a subsonic heavy bullet to open up
Go to each website and see for yourself.
They are pricey but they also drop deer -- I know others will write in that they have good luck with x or y brand but when I spoke to head of technical dept at Sierra and Hornady and Barnes they all parroted the same thing--"our bullets are designed to open at much faster velocities"
Your biggest problem will be getting a subsonic load to work your action-assuming you are on an AR platform.
I have a custom made 300 Whisper-aka 300 BLK, bolt and do not have the concerns of enough gas to work the action. I load for 1050 fps and then focus in on accuracy. Currently using Lil'Gun to great success. Have used AA 5 and AA7 and H110 but never got my gun to "appreciate" those powders with giving me both subsonic and accurate loads.