It's almost certainly at least a bit leaky; glass carboys tend to let a surprising amount of gas get in around the bung, which never seals quite perfectly. Tha

It's almost certainly at least a bit leaky; glass carboys tend to let a surprising amount of gas get in around the bung, which never seals quite perfectly. That said, even if it falls short of a 100% perfect hermetic seal it's still darn impressive.