I think you need to press the right trigger once to zoom in - it's not zoomed right in close though !
The sight then wobbles around everywhere, so when you think you've got him in your sights, just keep shooting - I think I shot him after about 5 shots.

Another tip for this level - Before you kill the target, go around the side of the ledge, look down and kill the dogs below - it helps for when you escape. When you have killed the tardet, follow the stairs all the way down and do into the courtyard where the dogs were. Now shoot off the lock on the steel doors. Now drop all your weapons and walk out the door. You'll see why.

I dont think you can Mate, Sorry, you'll have to use dead reckoning for the Mission with the Jail Assassination... And if possible could you help me with my Question?

-The Doc