I think it depends somewhat upon the time of year. In late Spring, when it's really too hot to find hogs out during the day, I've seen hogs in daylight in times of light rain. It cools them down enough so that they'll start moving and searching for food.
I haven't seen them out in heavy rain, but then I'm usually not out either if I have a choice.
You can never have too much ammo — unless you're swimming.