"When you absolutely positively have to kill everyone in the room in the shortest amount of time."
(note open bolt)
Because a MP5 is one of the best close combat weapons here is. For going into houses it is one of the best.
The Germans made a couple other close combat weapons for going into houses called the Panther and Tiger. They were used back when the safety of the house and occupants was of less concern, but they offered better protection for the user. Their intended use was longer ranged open field warfare, but they worked in towns as well.
They also invented the assault rifle, the Sturmgewehr 44.