In defence of the Mythbusters, they can only judge a "myth" based on their experiments. They didn't blow a barrel up during the experiments, so based on the evidence they had, they had to call it "busted." I question their methodology, though.
Regardless of what our firearms guru left in the barrel, that is a bad day. You lose a gun, nearly kill yourself, and now have the knowledge that the video your "friend" took of it will soon be on the internet. Forever.