i vote for the PLR16 kel-tec(if you can find one), its affordable, short barreled 223, so itll be easily maneuvered in the house, and still keeps a decent velocity for the round, unlike the ar pistols, its eeps it around 2600fps
http://www.keltecweapons.com/our-guns/plr-16/pistol/
a 223 krink pistol is another reasonably priced option, but i am unsure of its velocities
also look draco type pistols in 762x39, 8inch barrel puts out 1800fps with a 122gr bullet, plus they are CHEAP and reliable
http://www.budsgunshop.com/catalog/product_info.php/products_id/69679
i love lever action rifles, i am buying one this weeend, but i want a longer barrel to get some good power out of my 357s, but for my house which has a small hallway leading from bedroom to 3 diffeerent points of entry, i just dont have the room to swing around a gun with an 18inch barrel
this is the view from my bedroom, the door on the right is the basement, which totally bloccks my door when opened, around the corner on the right is the front door, and where the cats tail is, is the kitchen with a door to the deck
with a long barrled weapon, i would have to expose my self to pretty much every door in the house to get around one of those corners
so i guess it just depends on your living situation, if my room was upstairs, i wouldnt mind the length, but i feel better with a shorter firearm