My own experience suggests this is not so simple. I have been shooting both external hammer doubles and internal hammer doubles and O/U for decades and notice no difference.
I started life with right eye dominant and shoot from right shoulder. For last decade or so my right eye has deteriorated so much, in ways that cannot be corrected for with lenses, that I now can only use it to assure that I have mounted gun correctly. I now rely on left eye for everything else and still wing shoot from right shoulder, although all other shooting has switched to left shoulder and left eye. My wingshooting success in field and at trap and sporting clays has remained rather unchanged. I attribute this ability to adapt to failing master eye to having always shot with both eyes open, even with hunting scope sights.
Back to external hammer guns. I have always found that I have to cock both hammers to keep uncocked hammer from interferring with my "sight picture". This means that when trap shooting, I always cock both hammers. All my external hammer doubles have hammers that are well below sighting plane when cocked. The view down barrels is really same as with internal hammered doubles.
Niklas
