Guys, correct me if I'm wrong, but I would think that the reason a shotgun with buckshot is seen as an effective tool in these situations (although maybe it really isn't) is because it gives you the benefit of having multiple projectiles to fling at a small, fast moving target. Although I agree that slugs would have more knockdown power, they are just a single projectile and thus provide little margin for error. You may as well be using your big bore double or bolt then, no?
Ontario, the problem with the (shotgun) multiple projectiles is that in reality they are pretty small and lack the weight and velocity and very directed ...I dunno what to call it technically...force that a rifle bullet does. Hence their poor down range knock down /hitting power!