Subsonic are a real hat trick when it comes to cycling an AR.

There are only a few powders that work in 300 BLK and the rifle can be finicky. CFE Black, A1680 and Blackout (Shooters World). Keep in mind the primary purpose of 300 BLK was to shoot a 220gr pill subsonic. It shows you how finicky this can be. It’s a balancing act of having a powder slow enough that the gas pressure is high enough at the port (~15kpsi) to cycle the gun but fast enough to actually burn.

Then there’s having a rifle with a fast enough twist to stabilize the projectile at a subsonic velocity. So you can go with a heavier projectile but then it keyholes because it’s moving too slow and the twist rate isn’t fast enough to stabilize it.

I don’t think I have ever seen a 5.56 subsonic cycle a gun. Doesn’t mean someone somewhere hasn’t don’t it, it’s just not an easy task. Then there’s an issue with a 90 grain bullet having about the same muzzle energy as a 380 ACP.