I've played in the rain a few times as well, and some of it pretty heavy bursts. My MP5 has lasted no problem, not a patch of rust or corrision (or even discolouration of metal parts at all, even though my gun sat in it's case for a few days after. Exact same with my CA M24, nada!!!)
Apoc's M-16 series though (SR-16, M733 and G&P SPR), and his C-mags, have either rusted or gotten what looks like white salt stains on a number of metal parts. Just put a light coat of oil on your parts beforehand just in case and you should be ok if you wipe down well after the rain..