Well I'm going to go out on a limb here and tell you that I never cover my ML barrel when it is raining and I've never had a failure to fire. Maybe its because when it is raining, my ML is usually sitting snugly in my gun safe waiting for the sky to clear!
But if on the off chance that I would be somehow caught out in the rain, I would cover the muzzle with a "X" of electrical tape that I keep in my bag.