223 (5.56X45) and 300 WSM. Both fit in a wide variety of actions and rifle types, both have a wide variety of bullets, both shoot pretty darn flat, can be used in AR platforms (ar15 and ar10, respectively), can be used in target applications from 100 to 1000 yards, and can be found in almost any local Wally world in a pinch. (Yes, I have seen both recently in Walmart from Florida all the way to Washington state). With the exception of the big bears and the elephants, which most of us will not be culling on a routine basis, there is little else that you could not do with these two.