Rain doesn't seem to be giving me performance issues what-so-ever. Been playing on servers that have had it all day without issues relating to the rain specifically.
It's possibly related to another already noted issue in regards to old gear from previous FPS drops. Also could be related to the flickering issue on the M4s.
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System specs:
AMD Phenom II X4 965 Black Edition Overclocked @ 3.8Ghz (2400NB)
GIGABYTE GTX 660 OC 2GB
Kingston HyperX Blu 8GB DDR3 RAM @ 1600Mhz
Seagate Barracuda 500GB 7200RPM HDD
ASrock 970 Extreme3