Besides, what self-respecting sniper would use a 5.56mm weapon?
The 5.56mm round does a fair amount of damage when it hits but it just isn't a good choice for snipers. It lacks the range or penetrating power to do the job consistently. Something more on the order of a 30-06 or .308 (7.62mm) would be a much better choice, unless small weapon size is a major selection criteria.