I bought bungs and had the same issue (I brew one gallon and 38mm screw caps are so cheap and perfect that bungs are useless) but as long as you insert the bung for a few minutes and keep pushing it down, it will eventually mold to the shape of the carboy and not rise...trust me, I freaked out when I first tried to use a bung and that happened, but as you try to use them more, they will stay without an issue