Obviously, the final cause is recoil, since high pressure loads of negligible recoil, like the .22 long rifle, will never cause a well designed and well fit double gun to shoot loose, or, at least, I don't think they will. The less than final cause is poor fit or design, or looseness before the testing period. A well designed and fit gun will never, in our lifetimes, shoot loose with any reasonable use of reasonably loaded ammunition. In fact, an exceptionally well designed and fit gun may never shoot loose when used with any quantity of any ammunition that will not blow the barrels. I don't know the details of the Boss gun that was used so extensively, but that would be an example.