In most of the gameplay videos it shows mostly just stock weapons being used. A LOT OF IRON SIGHTS. You never really see silencers, scopes, or different magazines being used on the weapons in the videos. For a game that boasts how customizable things are in the game, they sure aren't showing it in their gameplay vids.
This has me worried about the kind of barriers that are in place to impede your customization options.
Up to 4 attachments can be attached to a gun at once. This includes silencers, scopes, grips, muzzle breaks etc.
http://www.youtube.com/watch?v=OphHeNA-kDA. Skip to 1:07