It ain't the cartridge or the rifle, any of the cartridges listed so far in this thread will do the job if the guy behind the rifle is up to it. A .243 is more than adequate for well past 300 yards if you know what you are doing.
True, but there is a point where any given cartridge just "runs out of gas" and becomes inadequate despite the shooters ability. For some cartridges it may be 150 yards, others may be 500+ yards. Better to err on the side of caution and choose a cartridge that allows some margin for error (which all but the finest/luckiest marksmen needs when shots get past 300 yards in field shooting conditions). When one selects a cartridge, they must accept any limitations inherent to that cartridge simply because a cartridge that is on the light side for a given game animal will only get lighter as it goes down range. No sense going undergunned, perticularly when ranges are expected to be long.
Mike