H4895 might do it, if you use something heavy like the Hornady 160gn RN or Woodleigh 160gn RN. 6.5CM will be tough to get subsonic without using TB- 308 has the advantage of 200+gn bullets to slow it down.
He's pushing the 160gn to 2500 FPS still, that's a big ask to slow down so much without using a special powder:
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300 BLK is the easy button for subsonic, it was purpose built for it and uses much more common powders to di it. I have AR, bolt gun, and a T/C set up for subs - no fuss trying to slow down a thoroughbred when there's a jackass at hand.