No, its not a trick of the light. The thicker rear portion of the cases is not expanding to chamber diameter. The bulge you are seeing is where the cases did expand. With higher pressure, the expansion would have continued further toward the head. Of course, whether higher pressure would be a good thing is an open question.
Another thought - if fired .303 brass is being used for this sort of experiment, make sure that none of the cases has incipient separations started. If this were the case, instead of a split case as shown above, the head could separate entirely. That could get interesting.
Another thought - if fired .303 brass is being used for this sort of experiment, make sure that none of the cases has incipient separations started. If this were the case, instead of a split case as shown above, the head could separate entirely. That could get interesting.
