First off, yes, I've been shot with a bb gun, but only by a pump action air rifle. Took a couple in the shoulder and thigh.
You have to be ridiculously surgical with a bb gun to do any significant damage. But, that's not to say you can't do any damage at all. With a CO2 bb gun, a hit in the throat from <10 feet out will slow someone down although it would feel like the largest bee sting in the history of this world.
But, if you're depending on it to slow down an intruder, it'll make them upset if nothing else. Not to mention, if you're shooting them with a bb gun, that indicates that you won't have anything "stronger" in the house.
If it's intruders you're worried about, the best small gun to get is a 380. It's a pistol designed for self defense. If you have little experience with weapons, then a 38 revolver would do (just point and shoot) and it will leave a significant hole where flesh should be from less than 30 yards out. Both can be considered "girl" guns.
We could go deeper, but I don't think it's appropriate for DISboards to discuss such a thing.