From my experience with CQB, and a 9mm pistol round in an MP-5 or other sub machine style rifles, are very effective. The great use and the reason why they are so good in a close quaters situations is they are less penitrative than say the 5.56. You can hit your target get good expansion, kill your target, and hopefully not volley some copper into the next room or two rooms. Your nieghbors may thank you too.
So bad guy dead in hallway, hopefully family safe in room.