The only place silencers actually go pffft is in the movies. In real life the level of sound suppression isn't all that significant.
It's over a 30db reduction on my AR using mine.
It's certainly not like the movie "pffft" with regular 5.56 rounds, but they sound like weak .22 rounds.
However, we loaded some subsonic rounds and I didn't even know they fired when I pulled the trigger because they were so quiet.
That being said, a 55 grain subsonic .223 round is pointless as it doesn't have enough energy to cycle the action.
It would be cool to see 9mm subsonic fired through an Uzi or MP5 and compare it to the movies.
edit: Just saw the topic mentioned .50 cal... I saw some video of a bolt action .50 with a suppressor and it was pretty quiet, but I don't remember where. If I find it, I'll post a link. I couldn't imagine a semi-auto .50 being quiet at all.
Actually -- one of the first links through Youtube:
http://www.youtube.com/watch?v=MzHzQE_2OgY Not the one I saw, but this is quiet.
One more:
http://www.youtube.com/watch?v=vlhrMKyLz1Qmir
