This is for my non-gun readers. The 5.56mm NATO round (SS109) leaves the barrel of a M-16A4 at 3100 feet per second.
Winchester is about to put on the market a .17 Winchester Super Magnum that goes 3000 feet per second.
Do you know what that means? It means that you're almost getting combat rifle performance out of a rimfire round (spare me the difference in bullet size and weight...you know I'm talking shot placement and a flat shooting rimfire on a nice trajectory out to distance never seen in a bullet of this type before).
Read about it here at Guns Holsters and Gear.
NOTE: Lets assume a modest effect range of 200 yards on a windless day. This round has applications for urban sniping. Even without a suppressor its report will be much less than a 5.56 or .308 and with the speed that its going with a proper bullet core it should be able to penetrate at least 1st gen PASGT helmets.