Someone was thinking outside the box on this one. FN-90
http://www.youtube.com/watch?v=0C0dmGeE4MI
http://en.wikipedia.org/wiki/FN_P90
This "selective fire personal defense weapon", or "submachine gun", or "compact assault rifle" has been around for 20 years, and I just noticed it. It comes with longer and shorter barrels, has little or no "climb" when fired even on auto, and a 50 round magazine which doesn't stick out all over the place. The "bullpup" design gives it a relatively long barrel in a short space.
If the utmost in concealability is not needed, this is what I'd want in an up-close firefight. The 50 round magazine and ease of handling would be the determinants for me.