Greetings CarlsenHighway,
IMHO, nothing is waterproof. But I’ve hunted is some pretty driving rainstorms over the years. One I remember it was coming down so hard visibility was down to about 15-20 ft. Powder has remained dry. Horn for the the most part is impervious to moisture, hence there use for powder. But again the weak links in my opinion would be with the installation of the base plug and its material, and however the pouring spout or type of pouring spout is installed. I find a simple friction fitted wooden plug ( like a fiddle peg for example) in the pouring spout hole has been more than adequate for the decades Ive used a horn.
This has worked well for me in the past. I would not go as far as to submerge one of my full horns in water. But as I stated Ive weathered some serious rains with no issues. Im sure an expert will be along shortly to contradict all I have written, and thats fine too.