Subsonic rounds may have already been discussed as they were used in previous Arma titles. Subsonic rounds would complement all rifle suppressors as in the real world there would be a supersonic "crack" or report if you will due to most rifle rounds with or without a suppressor. Muzzle flash would still be lessened considerably.
However there would be repercussions to such a benefit of the rifle truly having a lower decible count at a closer range. Most rifles cannot cycle a round fully as gas is lessened for subsonic propulsion and needs to be cycled manually by the user in order to chamber another round. The velocity of the bullet would slow and be effected by gravity severly. Thus reducing effective combat range. My solution would be to put an animation in game allowing the player to load a magazine of subsonic munition into rifle of choice, fire a round, then animate player moving bolt back and forth to eject old and chamber a new round. From what I have seen in my short time playing and reviewing news and updates the M320 LRR .408 already has a manual fire with a progress bar between shots. During that time there would be no option to fire another shot until finished or until animation is finshed if that part of the in game "HUD". My apologies if this subject has come and gone from the discussion table. I only hope to help bring about the realism that is upheld in this game's development. Thank you for your time.