I seriously doubt if shotgun pellets will be deformed after leaving the bore but I do recall this happening to a ML rifle which was tested years ago by the NRA. It was fired with pure lead round ball & both muzzle & down range velocity measured. With all powder charge increases the muzzle velocity increased but at some point ( with heavy charges) the down range velocity dropped. They accounted for this from deformation of the ball creating increased drag. Part of this deformation may have occurred in the barrel but they felt part of it was due to the force of air drag.as well. This was of course from a much higher velocity than obtained in a shotgun.
