In his book "The Modern Shotgun" Burrard cites several verified cases of Shot Balling. One case in particular injured a beater on a driven shoot who was Well out of the line of fir off to one side.
This is much more likely to occur with card & filler wadding as they do not expand enough to seal between the end of the shell & the bore itself, particularly as the chamber transforms into the forcing cone.
This is far less likely to occur with modern "Obturating" wads, of which virtually all plastic wads are. I would personally be very cautious about using short shells with card & filler wads or even in guns having long forcing cones for that matter. The older guns with the short cones were designed so the forward end of the wad would be tight in the cone prior to the rear clearing the hull mouth in order to bridge this gap.
