Probably one of the major can of worms you could open, some makers say its on purpose to accommodate 10m target shooting or that its a result of machine wear in high volume factories. It isn't restricted to break barrels either but may be more common in European rifles, if the barrel isn't visibly bent then its a matter of getting proper scope mounts since I don't believe it affects open sights as much. This is just my opinion quickly made from what I recall reading else where and its not something to really worry about, usually it can be overcome by investing in a droop mount. If you don't mind spending the money.