Considering that we are coming up on a time of year where precipitation is likely, I went digging around for some old info I used before, and found one heck of a gem:
http://airsoft-surgut.ru/video/a_328x288.wmvThat aside, I have gone airsofting in some really bad conditions; Once in a typhoon for a 2-day op with my first AEG (110mph wind, definitely bucketloads of rain), another in the middle of what I could best describe as the worst possible terrain available in Florida (marshland). Both times I have had AEGs and they have gotten seriously wet.
In the typhoon op, we spent most of that 2-day op hanging out on the side of a cliff (there was a cubby hole in the side of it that we rapelled down to, in order to conceal ourselves). It was not dry in there by any stretch of the imagination. Most of the 2 days we had that op, we were out of the cubby hole and raining attrition on the other team.
The AEG I had was a TM MP5, regular battery in it, nothing fancy. It survived without even one complaint from the gun.
In the Florida op I attended, I was actually laying in the marsh trying to evade the OpFor. Most of me was submerged, as was the gun. I was in there for about 45 minutes, give or take. Not sitting still but crawling through the mud and slush.
I was using a TM 733 for this op. I did experience a couple of jammed BBs (due to silt in the barrel) but other than that, it also did not suffer one bit.
Prior to both ops I did silicon the guns down to make sure they at least had a little protection from the elements. But neither of those guns failed as a result of being very extremely wet (or in the case of the 733, submerged!).