I guess I've never shot at the range before in the rain. I had 2 guns with me and my accuracy was terrible along with my grouping. If I had to guess I would say that rain would deflect the bullet's path to some degree.
I have shot many long range matches in rain, sometimes so bad it's hard to see that target, and not only does it have no effect on my drop, but it's easier to see the wind, and there is no mirage. I've shot some of my best 1,000 yard scores in the rain.