I think there is only minimal effect (light rain anyway). But the first thing that is lost is scope visability; thus its visibility that usually stops shooting (at long ranges anyway).
Here in New Zealand it's not uncommon for F-Class/NRA target shoots to be hulted for rain (that's why NZ is so green!). Sometimes this happens mid-way through a string. At our range in Hamilton the rain will often arrive at the target end and then move up the range to the 1000 yard mark. Thus there are times when you are definitely rain-shooting. In the hand full of shots I have fired through real rain from 1000yds I have not actually seen any significant change to point of impact (within say 1/2 moa). Soon after though it gets heavier, you loose scope visibility, so ya cover the rifle with your ground mat and head for cover.
That is my small amount of experiance in this area. Am intersted to see others views.
Maybe the answer is that at supersonic speeds the shock wave that gets pushed out in front of the projectile actually pushes away any rain drops???