I know this doesn't have much to do with the thread, but it's hard for me to let people think things about guns that I know aren't true.
Backflips are an exaggeration! The perception that most people have of how a person reacts when shot are gathered from Hollywood rather than reality.
As far as penetration goes (huhu, I said penetration): A .45acp round has less of a chance of passing through a person than a 9mm or .40 caliber round. It has a larger face, travels slower, and with hollow points, expands in such a way as to make it very unlikely to go through a person (at least on a torso shot). This is also where they get their deserved reputation as manstoppers. The size of the round, and the fact that they stop inside the target, means they transfer all of their energy into the target, which is far more effective at neutralizing a scumbag than hotter rounds passing right through.
Also, as far as police departments and what guns they use: Most .45's have grips that are too large for some people, so most police departments who allow them, do so as an option, not mandatorily. Other departments who do not alow them, do so for many reasons. One is ammunition compatability. Say officers A+B get into a shootout. Officer B runs out of ammo, but can reload using his partners magazines. Other reasons include the public's misconceptions about the round (people think "oh they use .45's, they're just looking to kill people" - uninformed people, but an issue nonetheless), issues with officers being accurate enough with them, etc.
I can point you to a couple really interesting books/studies that explore this kind of thing in depth if you're interested.
