Maybe I don't fully appreciate the ability to change ammo types at will in the middle of a magazine, and no I haven't had formal defensive instruction with a shotgun. I will admit that I don't know what I don't know.
As to multigun competitions being completely unrealistic, I agree. That's my whole point. I think that they would do better to be more grounded in reality. I figure that if more common people have mag fed shotguns that 3gun comps would treat reloading the shotgun like they do reloading pistols or reloading rifles, and not as the make-or-break skill they do now.
