I believe that all barrels, even singles, exhibit muzzle "flip"

Muzzle "flip" is more of a problem for those of us who shoot SxS rifles and has to be really taken into account there. The right and left barrels do not flip the same.

A lot has been written about flip and you can find it on double-rifle sites. SxS shotguns have the same phenomenon but it is more easily demonstrated when you have a single bullet coming out of a barrel as opposed to a shot pattern.