Since this doesn't happen when shooting clays which is a higher volume shooting than in the field I would investigate further what the differences in your gear - range versus field. Perhaps you wear a shooting glove which gives you a better grip at the range? Or if you are shooting premounted gun at the range versus mount and shoot in the field you are changing your grip. Your field clothes are going to be thicker usually than range clothes which was mentioned earlier. The solution would seem to be in the differences.
