The problem with subsonic .223 is... to have the utility of 300 BLK you would need the highest common twist (a 1/7 twist) to stabilize your subsonic round, you need it to cycle a "stock" carbine gas AR15 with standard carbine spring and 3oz buffer, and it needs to fit in an AR15 magazine. The 300 BLK cheats with a pistol gas tube length.
This is very difficult to achieve. I believe there was a ~100 grain powdered construction bullet at some point that was supposed to function and cycle semi auto carbine gassed 5.56 AR15s but it had horrible accuracy and terminal effect while being uber expensive and not widely available. The ATOMIC bullet yawi g in flight ensures 100% that accuracy at 50 yards and beyond will be ridiculously bad.
300 BLK in contrast has the 190 SubX which is sub MOA capable at 100 yards, is widely available, reliably expands and penetrates 14-18", cycles without issue, and is only moderately expensive.
A 77 grain .223 bullet is not heavy enough to run subsonic reliably with terminal effectiveness. OTM match bullets that yaw without fragmenting are horrible for terminal effect. Better than .22 lr 40 grain round nose lead bullets? Sure, but that should not be ter termial effectiveness standard to measure against. CCI Standard .22lr is subsonic, will actually just barely penetrate 12" in gel because it just pokes a .22 hole and does not expand.
You need expansion or fragmentation for subs to be terminally effective... meaning to be AT LEAST in the same realm as .380 or 9mm expanding hollow point handgun ammo. You need 12" mini.um penetration minimum if such a round fragments or expands.
Noone will release a cycling subsonic .223 that only works in pistol gassed 7.5" barreled uppers. To get enough weight to make the round work, yet be short enough to stabilize would require something exotic like a 100 grain round with a tungsten insert. To get terminal performance. You would have to have stability for a straight frontal impact and the something with 3 copper petals that expand wide, backed with a tungsten slug behind the copper nose cone. The tungsten might make the round short enough yet heavy enough to stabilize and function, but would likely define the bullet as AP handgun ammo to the ATF... so again, not commercially viable.
Accurate, cycling, subsonic .223 ammo that has better accuracy than 9mm with at least the same terminal performance as 9mm is just not going to ever be a thing I am afraid.