I would say they could have used 300 blackout ammo round, 7.62x35mm. It is subsonic round, very silent but has more impact power then nato standard round 5.54x45mm, but has a short range of being precise, 200-300m, based on russian vss vintorez(9×39mm) that was used in Afghanistan for it s silent characteristics. Cs said in Dallas attack they used subsonic ammunition also. And going for a head shot, yes, I would say it was distance of about 150 200m, maybe more so that s why it was a miss, but still as said professsional would know that and would not miss, even if it can happen, only if there was some providence in action. But by using silent ammo it is about not getting caught so it has similarities to previous black ops attacks, using also supressors to make it more silent and not having muzzle flashes to see the direction of fire, but with stronger calibers there are also natural ways to mask yourself depending on enviroment. And why to use assasination as said here when they can use other not so overt methods.
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