Rangrz: I subscribe to the school of thought that a handgun is for shooting your way to the rifle you should have never left in the first place. I absolutely agree that handguns are underpowered and more difficult to aim, so when something goes bump I grab a Kalashnikov. With an OAL of 26" its plenty maneuverable enough indoors, and if a 7.62x39 round isn't enough to stop whatever made the bump, I'll just give it another shot (30 more shots, to be exact).
The problem with using a rifle in this scenario are, first and foremost, overpenetration. Using something in 5.56 diminishes this problem, but does not make it go away entirely; while I do love my DSA FAL, c'mon, you know a 7.62x51 round is literally overkill at contact range, and unnecessarily putting his neighbors at risk, and same would apply (to a slightly lesser extent) to my x39's if I lived in an apartment.
Another problem, one frequently overlooked, is just how loud rifles are indoors. Last time I discharged a rifle in an enclosed space w/o hearing protection I did some serious, permanent damage to my hearing (which I somehow failed to notice at the time), and that was only a few "intermediate" rounds- I don't even want to imagine what the effects of 20rd's of .308 would have done.