Like others have said...600 yards is a hell of a shot...beyond my skills with any gun I own. Anyway...to the question...300 WM is a tried and tested caliber. If you are not a reloader...buying 300 RUM ammo may require a second job! 300 WM ammo is far more prevalent, has more bullet choices and is far less expensive...oh and it will be kinder on your shoulder. To get proficient out to 600 yards is going to require some...hell a lot...or range time, hence a lot of ammo (so back to the cost thing)...and a 300 WM will kill as well as the 300 RUM.