Newbie Question about the Debate on Caliber Effectiveness

Hey all! I've just recently gotten interested in CCW and firearms in general, and I've noticed a fair bit of debate as to what ammo is "effective" for CCW. Some people, for example, argue that .380 ACP is the bare minimum for a defensive carry firearm. Likewise, I just saw online someone discussing whether .30 for a rifle was effective for home defense.Now, I've never been shot before. I'm sure many of you haven't. But it seems like it hurts quite a lot, whether it's a .22 or a .44 magnum. I know some rounds will do more damage, some offer better penetration, etc. But it seems like even the least powerful bullet would hurt quite a lot entering your body with enough velocity to break through skin, bone, muscle, etc.So, my question: Could someone help me understand the debate on caliber effectiveness? I think can understand it to some degree. I understand that different ammo types and different calibers will do varying amounts of damage, but I can't see how an attacker could "shake off" getting shot by any one of them like, "Bah! That was nothing! Should have used a .44!"Thanks in advance!Edit: Some formatting and such. via /r/CCW http://ift.tt/2aMxH4q