I had a Beretta 687 that did the same thing. I took it back to where I bought it, they were also a Beretta warranty dealer and said they couldn' t do anything about it. I sold it right away because it also pounded into my face when I shot. This was years ago and I' m pretty sure they have a fix for it by now. Call Beretta and see.
By the way, what shot are you shooting through that gun for waterfowl.