Given the same weight & same powder charge, a load of shot will produce a higher pressure than will the round ball This is due to the increased friction of the shot charge. Prior to choke boring it had been standard practice when shooting a round ball from a shotgun to use a fairly close fitting ball. If this size ball was fired through a choked gun it would have a diameter larger than the diameter of the choke & likely result would be a split muzzle.
A round ball can be fired from a choked gun with complete safety, As Long as the diameter of the ball is smaller than the choke. Accuracy may suffer but no harm will come to either the gun or the shooter, given Proper Loading.
The misunderstanding of this was one of the major reasons for changing the marks from "Not For Ball" to "Choke" As I recall most proof laws required one of these marks, depending upon time frame, on any gun having 0.2mm (0.008") of choke or more. It was I believe optional on a gun with any lesser amount of choke. A cylindder bore was of course unmarked.
