No discussion of this subject would be complete without reading Bryan Litz's essay Maximum Effective Range of Small Arms http://www.appliedballisticsllc.com/ind ... _Range.pdf
It all hinges on how you define "effective", is a hit on a fig 11 good enough or are you going for a headshot on a rabbit? Do you need to do it 100% of the time, or are you prepared to accept a hit probability of 90% say? Can you adjust for conditions in the field or do you want to "point and shoot"?
His rather surprising conclusion that for a 6" target (for 100% hits under worst case conditions) the maximum range for an 80gn varmint bullet fired from a 243 is 248.5yd was roundly poo pooed last time I mentiooned it on this board, but with no actual argument against his methodology, just that it was "ridiculous".
He's a well qualified guy, chief ballistician for berger bullets, and a champion shooter, I'd take notice of what he said unless I had a good reason not to.
You may want to read "Extending the Max Effective Range" as well http://www.appliedballisticsllc.com/ind ... ticles.htm