300 Black Out should be added as ammo type option for all 5.56mm Assault Rifles. 300 Black Out barrel should be added as optional barrel for all 5.56mm Assault Rifles. 300 Black Out is subsonic 7.62x35mm ammo. 220 grain 7.62mm round is placed in a 5.56mm shell casing with the opening on the end made bigger so that the 7.62mm round would fit in it. In real life it is only the barrel that is swapped out and everything else is kept the same when switching from 5.56mm NATO to 300 Black Out. The amount of ammo in the magazine remains the same at 30 rounds. Add AK-9 that uses 9x39mm subsonic ammo. AK-9 uses 20 round magazines.
Ehhhhh, you've got a few technical things wrong.
It shouldn't be an option for all 5.56mm weapons in game, while a .300BLK barrel is available for the G-36, I'm not aware of any military units using it. Nor would it be used on a 20" barreled rifle like the M-16.
Even though it's "just the barrel" that needs changing out, this is not a drop-in modification for an AR family rifle. It's a good hour of work. USSOCOM has deployed a few .300BLK rifles and done so by making it a complete upper receiver system that is swapped onto an M4A1 or Mk.18 lower. Especially since the .300BLK cartridge performs so much differently than 5.56mm, it would probably make more sense for it to just be a totally new gun in game.
.300BLK isn't exclusively subsonic. There are subsonic and supersonic ammo variants. This would be a pretty cool game mechanic really, choose between quiet ammo and more powerful ammo.
Anyway, I'd love to see .300BLK rifles in the game, but I don't think a barrel modification for existing weapons is the way to do it. There should be a whole new weapon chambered in it.