1.MP5 imo ain't the best to use for hip fire.. as from my experience..I don't get many kills like that with this gun, altho i use it as my stealth class weapon, I always try to aim down the sight.
2.P90 this is most def. the best hip fire weapon imo. No matter if ya got stopping power/double tap/steady aim/bullet pene, this gun should give the best results.
3.Mini-Uzi.. hmm I haven't really used this weapon lately, but what i can remember it has a wide hit diameter, so this weapon should most def. have Steady Aim, and aiming down the sight..well this weapon just jumps so much..=D so you couldnt aim much anyways.
4.M16A4... This gun seems to have great accuracy even when used to fire from hip..but since it fires 3 bullets at a time.. it may not be enough for you to kill your enemy before she/he gets you, but if you manage to get the hit, 1-2 hits and next please.
5th and finally maybe M4 Carbine...no comment on this one..maybe only to say that it is better used aiming down the Red Dot on this one..=>killing machine with its accuracy and rate of fire.