[merak]

You can't totally suppress a highpowered rifle like the 308 or 300 winmag. You can get rid of the muzzle report but the sonic boom is still there if not directional. For real silence you have to have subsonic ammo and they have heavy subsonic ammo and rifles to fire it but, to my knowledge, you cannot load your normal caliber rifle ammo subsonic.

[E.Allen]

I believe the suppression is used because it makes it much more difficult to localize WHERE the sound is coming from.

They're not trying to eliminate the sound, just disguise the location of the sniper.

[8mmMauser]

Engle Ballistic Research does subsonic 308 thumper, 180 gr. warrented with MOA accuracy. Also .223.

[Kadric]

There are sub-sonic .308 rounds. They are designed specifically for this use.