I have zero experience with slugs on game. Here in Canada the 1 1/4 oz Federal slugs have an outstanding reputation for bear defense. I stoke them in my 12.5" 870 Clone when fishing. After reading this thread I am starting to wonder.
The slugs "outstanding reputation for bear defense" - is sort of like one of those urban myths we hear about every so often.
You know, like one about the couple who went to Mexico and bought a rat, all the while thinking it was a chihuahua - that sort of thing.
I think the logic behind the myth, goes something like this - the hole in the barrel is huge, the kick is powerful - they must really flatten game!
The urban myth sometimes starts to fall apart, if just one simple question is asked.
"So, have you killed a lot of charging bears with these slugs?"
Some "facts" - are better left untested.