Don't know about the scope question, but I was thinking about getting a Barrett a while back and eventually suppressing it (yes, they make suppressors for them - AWC Turbodyne is one example). Suppossedly a suppressed 50 requeires no hearing protection.
Well, with a bolt-action, your OK. But with the semi, you would be creating an illegal SAW, since the suppressor is considered a very efficient flash-suppressor. The Barret already has the pistol-grip and detachable mag for the 2 allowable evil-features. The Barrett is definitely post-ban.
But, I understand, the semi has a bit less recoil because of the Browing-like barrel movement. Of course, the suppressor is also supposed to help with the recoil.
The noise it still there, though. So it would still be definite a crowd-pleaser, and cool as shit with the suppressor. You could always take the can off for the full effect.