Don, there's a zillion old threads about this, and plenty of well informed input. Do a little searching in the archives,you'll find what you're looking for.
Then, if there's still some questions, please post...
Are these things worth a darn? I've never found a good deal on them, and thus haven't shot any, but according to Louie Awerbuck all the brands he tried (granted, last time I talked to him was almost two years ago) crapped out by the fifty yard mark. Apparently the manufacturers use the standard slugs and simply drop the powder charge, meaning that the projectiles start destabilizing MUCH sooner.