RE: ultimate defense gun
If you're shooting at someone at 100 yards, it's unlikely self defense unless he's got a rifle, and there aren't too many crackheads conducting fire and maneuver assaults on residencial homes from down the block these days.
I would say that I'd probably go with a pump 12 gauge loaded with 2 3/4" OO buckshot and a IC or Cyl Bore choke with rifle sights. But if you think you really need to reach out 100 yards or more, I'd go with an M4A3 style Ar-15 with both a holographic sight and iron flip up backup sights and a collapsable stock in the collapsed position and the 16" barrel and short handguards. It'll reach out to well beyond 100 yards if you needed it to and is compact enough for CQB. Use either a varmint bullet (frangiable) or a long FMJ like the M855/SS109 NATO ball to limit overpenetration and you're all set. And yes, contrary to the intuitive sense for the 5.56 NATO ball ammo, it isn't a great penetrator because it becomes unstable and tumbles the instant it hits anything substancial, particularly flesh.
Mike