Here's another way of looking at it. Could be totally off of the mark, so take it for what it may or may not be.
Both the raindrop and bullet are both being acted upon by the exact downward vector (gravity), so could a raindrop really fall onto a bullet in the first place? They are dropping at the same rate. So in my eyes a raindrop couldn't effect the bullet on the vertical plane.
Now if the bullet were to potentially hit the raindrop on the horizontal plane I guess you could think of it as an arm swinging towards a feather. A direct hit would have some type of an effect but the time interval and force of such hit would be minimal (obviously the longer the bullet travels in such case the more off course it could become, but between all the factors stated previously you shouldn't see much change.)
