Shooting shot from a slug barrel is far from ideal, but if that was all I had I think from my limited testing I wouldn't feel to bad about it at very short ranges.
I tried a few loads through a Mossberg 500 with a 18 1/2" cyl bore barrel and with a 24" rifled barrel. What I found was surprising. The biggest downside is a MUCH more open pattern, but depending on the distances in your house this may not be a problem, it may even be a plus.
At most "in the home" ranges, a heavy field load and 00 buck loads would still be effective at under 21'. I found full shot wads did more to open a pattern then just a fiber wad like in the S&B 00 buck loads. All 12 pellets of the S&B 00 buck load was on a B12 torso at 21', barely. Remington reduced recoil 00 buck loads had 7 out of 9 on target. The plus side to this would be an easier hit on a bad guy and a reduced chance of over penetration. It's pretty well known that a 00 buck load from a smooth bore at short range acts a lot like a slug. The rifled barrel keeps this from happening at short ranges. I really doubt that a single 00 pellet would carry much power through a wall, even an interior wall, so the extreme spread could be a plus.
Heavy field loads (#4 shot, 2 oz, 3" turkey loads) open enough to evenly cover a B12 target from waist to neck, shoulder to shoulder at 21'. Again from a smooth barrel it patterns much like a slug, a 3" wide hole at 21'. The penetration of #4 shot on a BG many not be ideal but I think the area of impact might make up for it. Again given how quickly the load opens I doubt any of the pellets would be able to go through a wall.
Past 21' the chance of getting hits drop quickly, so you are pretty much limited to 21' or less using a slug barrel. Of course if you have to shoot past 21' use a slug. Given most homes I doubt a shot of more than 21' would come up. I did my shooting with a 12ga, I'd expect a 20ga might reduce the max range you'd have to work with because of the smaller payloads.
In the end if all I had was a rifled barrel, I'd use it, but it is far from the right tool for the job.
