Well... youz know those funny-lookin' conical hats Ashigaru wear? Me thinks that serves as an 'umbrella' of sorts in rain. Thus, you can shield the firing mechanism from the torrents and fire away! Of course, water in the barrel would make them spew in funny trajectories, kinda like spit does on baseballs... so me thinks accuracy would decline.

Interesting though... will have to research the impact of this on the mighty wet gunny wedgie army of doom...