30 cal. is my choice. For one rifle ... hard to beat the 30.06 Spr.

Not to lecture here but I have a pet peeve when it comes to caliber and cartridge. Lots of folks refer to a certain cartridge as a "caliber". This is very common and what I consider an innocent error. Caliber is reference to the outside diameter of the bullet. Cartridge is a descriptor of the case in wich the bullet rests. And the two are not interchangeable terms.

Example .... the 30/30 Win., the 308 Win., the 30.06 Springfield, the 300 Win. Mag, , the 30-378 ... etc. are all 30 caliber rounds. In this example, each "case" capacity increases from the "mild" 30/30 Win. to the behemoth 30-378. ... more powder ... which translates generally into capable of higher velocities .... and delivering heavier recoil! To me "caliber" simply does not tell the whole tale.

But you're talking elk at 600 yards ? That is a heck of a poke. My guess would be 300 Win. Mag. or 338 Win. Mag. .... something along that line.

I am off the stump. The 30.06 Spr. will handle bullets from 110 gr. up to 220 gr., with the 150 grain to 180 grain being about optimal. I have never hunted elk, moose or bear, but I read where with the right bullet the round can deliver a quick kill to these larger critters. But at 600 yards .... hmmmm?