there are Two main rifles the SCAR and the smaller SCARAB http://crysis.wikia.com/wiki/The_SCAR http://crysis.wikia.com/wiki/SCARAB
you can only attach under-slung weapons and more longer range scopes to the SCAR. the SCARAB can't use under-slung weapons, or the long range scopes, but can use a suppressor (proper term for a silencer), and the more shorter range scopes.
so SCAR more for assaults, SCARAB more for stealth.
this bugs me that you need certain version for certain situational and I thought the SCAR was going to be the jack of all trade weapon
also it could be you haven't found a suppressor yet, because you can only get attachments if you find them on guns you pick up. the starting rifle for the game is the SCARAB, and I found a SCARAB with a suppressor when I was making my was to the crash sight of the....second level or third level.
and a note suppressor decrees damage so keep that in mind when you fighting aliens.