Slugs. From what I've seen they are awesome penetrators(punches thru bulletproof glass), and you can get rifled barrels for the 870 to fire slugs accurately, would an 870 shotgun with rifled barrel firing slugs no longer be considered a shotgun?
Rifled bbl or smooth, shooting slugs or shot, it would still be considered a shotgun.
A very important thing to consider about using slugs for SD/HD is over- penetration. Don't want to be shooting through your walls into your neighbors house. And the 'kill range' of a "free flying', unobstructed slug can be several hundred yards.
You should survey your home/surroundings and then decide what load to use for SD/HD. Have never seen a pole on the topic but would guess that most people using a shotgun for SD/HD use some form of buckshot for two reasons:
1) buckshot does not penetrate as much as slugs but is still more then adequate enough for most SD/HD situations.
2) buckshot spreads giving it a better 'hit' ratio then a single slug.
Just an idea, how about an 870 with red dot sight, extended mag tube, and short rifled barrel for home defense? Too extreme?
Would work just fine if you do your part and is not to extreme.
Oh, and wouldn't firing 00buck through a rifled barrel damage the rifling?
No it will not. The shot is softer then the bbl. It WILL lead up the bbl rifling so shooting any lead projectile out of a rifled bbl. will require cleaning with a 'lead removing' solvent.
Bottom line: on shooting buckshot out of a rifled or smooth bbl for SD/HD would again, first include surveying the area you are trying to protect, IE: furthest distance you would be shooting in your home, and pattern your shotgun at that distance with different loads till you find a load that gets most shot center mass on your target, will do the job and not over-penetrate.
FWIW...Kudo's to you for asking the questions you have. Many don't and it show's much responsibility on your part.