Quoted:
I was wondering if one could here a 223 at that yardage. Its nice to know, when shooing steel, if its hit or not.
I've heard them out to 500 (75gr fired from 20" barrels).
But for a universal plinking round? 4 come to mind.
.22LR - cheap and probably the world's favorite for 100y and less. Can't realistically be reloaded.
9mm - inexpensive, reloadable, but 200y (and up) with a 9mm is more like calling in artillery...
5.45x39 - another cheap round (currently) that has some good BCs for those longer range shots. Cheap to fire, but a PITA to reload (see the 5.45 FAQ).
5.56 - Not as inexpensive as it once was, but it IS reloadable, reloads can be inexpensive. Lots of load data and options for loading.
IMHO the 5.56 is what you seek.