The US Army looked at this kind of ammo in the post WWII period. Ammo was tested, but in the end the 5.56x 45 was selected for the same reason - low recoil, low weight.
See
http://en.wikipedia.org/wiki/Flechette The 5.7 x 28 in military issue will do this as will old Russian ammo (steel penetrator)
My sister is a police officer - she wears her vest, but is more worried about bad drivers than creeps with “cop killer” magic bullets......
A couple of our local SWAT officers are alive today because they wore their vests. The suspect fired multiple rounds of .357 magnum at the officers. One officer was hit on the collar bone (covered by the vest). It broke the collar bone, but did not penetrate. The 2nd officer to a round through the buttocks (the lead is still in his butt) and a second round in the back (covered by the vest). The round in the back would have gone right through his heart if not for the vest. It still left a big bruise underneath.
It's a good idea to wear the vest. On average, you have a better chance of survival in a SWAT operation.