You can also use a small balloon stretched tight on the muzzle. Stays put like the finger cots do and you wont have to mess with tape. Real cheap- you can get them at the dollar store. Just make sure it's the small kind so it stretches tight when you install it.

I will throw a couple in my pocket when I think I might be in for nasty weather that day. I have used these in downpours with great success in the past. They havent affected my point of aim/impact, I have taken a few deer with them on in the past 5 years or so.