either use finger cots. Like a condom but will only cover a finger, works great, but if you are used to old army tricks, is your rifle too long to carry muzzle down?What do folks use to seal the muzzle when its raining and snowing? My 300 win mag has a brake on it and I am not sure if I should tape it all up, do the old army trick of putting a condom over the end of it or not worry about it. I haven't had a wet, sloppy hunt but have my second one booked and thinking I want to be ready if the weather turns crappy.
3M EARS foam Hearing Protector.
Roll one as if you were going to stick it in your ear but instead stick it in the muzzle. The ends are waterproof and it poops out when you shoot long before the bullet gets there! Not too many bores it won't be big enough in diameter to give a snug fit and will fit a .177 also. If you want you can cut them in half with your knife since both ends are waterproof.
I don't think sticking anything in the muzzle is a good idea. Not only is it an obstruction, something like a foam ear plug could absorb moisture, and cause rust inside the barrel where the plug contacts in a really short time. It might work well for you, but I wouldn't do it. It might prevent rain from getting in there, but if you drop the rifle, or stick it in the snow while walking, I don't see how it could prevent anything from plugging the bore (it would just push the ear plug deeper into the barrel).
Tape or other rubber cover over the end of the barrel will stop most crap from entering the muzzle.