What problems did these people you observed typically have on the pistol gripped stocks? Would these same people prefer a conventional stock rifle like an M1A to a pistol gripped stocked AR10?
You shoot a rifle and a shotgun differently. Thus what works well for a rifle is not automatically what works well for a shotgun. And it wasn't that they had problems because of a pistol grip, it was that they performed better with a standard stock for most exercises.