Re: how does rain affect bullet flight?
One of the primary differences that you're likely to notice is that your trajectory will be flatter, compared to your zeros from a nice, sunny day. This isn't due to the rain itself, per se, but to the lower barometric pressure that accompanies (or allows) the rain. I've seen changes of a minute or better between two silhouette matches, the first fired in a nice clear morning, the second later that afternoon as a front moved in and it began raining (gotta love Missouri). I had to come down a full minute to a minute and a half for the second (afternoon) match. Same gun, same ammo, rain (and barometric pressure) being the only difference.
As SBruce pointed out, water in the bore or chamber can be a real problem, but that's actually another issue. I'll add flash suppresors to the mix here, even if the water doesn't make it inside the bore. Those that don't provide adequate clearance, like a standard (not NM reamed) M1A/M14 suppresor, will give wild shots with the first shot, every time.