I have to agree with tankie that 9mm isn't much cop unless you can smell their breath. SMGs and pistols are only really useful in CQB, since a wet greatcoat will stop a pistol-calibre round at any sort of range.
Also, the calibre of a round doesn't have anything to do with how deadly it is. It's how the round is shaped or how it's designed. 7.62mm NATO FMJ will go right through your kevlar vest, tumble around inside you and turn your insides to mince, then bust out the back of you and take a fist-sized chunk of meat out of you. That's due to the shape of the bullet, the fact that it's a relatively soft lead core covered in very hard jacket of anything from brass to titanium to aid with penetration, and prevent the round pancaking once it hits you in the plates (in your flak jacket).
Now, a 9mm JHP round, despite being a "larger" round, would only penetrate about an inch at most, before expanding inside you and causing massive damage and bloodloss... that is, if you weren't wearing anything thicker than a string vest. That's due to the fact that a 9x19mm parabellum round has less propellant and is designed to stay in the target, rather than bust out and hit someone behind them, which is exactly what the police don't want to happen. The military, however, would consider that to be quite a bonus, taking one bloke down and getting his mate too.
The reason so many organisations use the MP5 is because it is reliable, accurate and can empty a magazine in less time than it takes to spit a mouthful of marmite out. At CQB ranges, it's a nice weapon, since at 5m max you can ruin someone's day with a quick burst. If whoever you want to slot is wearing body armour, something like a Colt Commando (M4, CAR-15, whatever you want to call the damn thing) would be used instead.
And don't underestimate .22 rifles, by mate shot himself in the leg with his a few years ago, hit an artery and lost a fair bit of blood. Also, Israeli hit squads use .22 pistols for assasination work, since if you shoot someone in the back of the head with one, the round will go in but lack the power to bust out of the front, so it ricochets around the head and turns the brain to bolognese.
Right. Back on topic?