Oh no, the more dramatic orchestras will use cannon indoors too. I was at a rather excellent concert some years ago, thoroughly enjoying the music and very much ‘in the moment’ as you might say, when I discovered this fact. I was seated up in the gods, and one of the cannons was, it turns out, on a gallery almost directly above me. They pretty much had to scrape me off the ceiling when that thing went off, and I don’t think my heart rate came down until I got a stiff drink in the intermission.