Two things, for 600yrds you don't need a magnum (I here the booing and hissing as I type), but a good cartridge such as a .308 Win, 260 Rem, 6.5x47 Lapua, 6.5x55 Swede and any long action such as a 25-06(115 or heavier),270, 280, 280AI, 30-06, will be more than enough to drop your intended animal. As far as a 243... it would questionable to 600, but not out of the question. You would need to use a 107SMK or 115 VLD which requires a high rate of twist that the rifle you are getting more than likely will not have. Give this article a read and I hope it answers some of your questions.
How Do I Know What To Buy?
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