I've shot a few matches in the rain. No significant accuracy issues through 1000 yards in my opinion. But lots of folks blamed bad shots on raindrops.
Biggest problem I've heard of regarding accuracy was when the M14NM rifles first came out. Their flash supressors were small enough that rain drops on their inner surfaces deflected bullets enough to lose accuracy. After the services reamed them out, the problem went away shooting in the rain.
But there were other problems with bullets barly skimming supressors on their way out, so the NM version was reamed out to allow clearance for that, to. There was even a gauge made to see if the supressor had enough clearance.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master