Just thought I'd throw out a different kind of suggestion as food for thought. Have you considered a Saiga 410? It's semi auto, light, dependable, inexpensive, and you can buy high capacity magazines for it. You can buy slugs or 000 buckshot for it and the amount of spread for buckshot at inside the house distances is negligible.
And don't make the mistake of thinking 410 buckshot is somehow weaker than 12 gauge buckshot. 000 buck is 000 buck regardless of the type of shotgun. It leaves the barrel at the same speed and has the same penetration from either. The only difference is you get fewer pellets out of a 410.
They might not be a perfect solution for everyones HD needs but I would submit there is nothing you are going to do with a 9mm or .45 carbine that you couldn't do with the Saiga 410. And unlike the pistol carbines you can buy the conversion parts if you want to add the pistol grip and collapsing stock.