I've had time to look it up and here's a link to a very brief explanation of barrel flip in a double rifle:

http://forums.accuratereloading.com/eve/...6481#2781096481

Same thing for a shotgun. That's why rifle barrels need to be regulated for a specific weight bullet/powder. Bespoke shotguns used to be (I'm not sure if they still are) regulated for the new owner's favorite load.