2 yrs ago local farmers were trying to fill their tags and asked for help up here in southern Alberta. I sent a 143 ELD-X into a cow at 600yds. I barely got a head turn. I saw, and heard, the bullet impact. Thwap!!! I was completely shocked. I had just taken a WT buck at 615yds about an hour prior. Dropped on the spot. All other elk kills from me have been with .30 calibers. 30.06 with 200gr. Accubonds, .300 WBY mag with 180gr. partitions. I know others have taken elk at greater distances with the same 6.5 bullet combination so I don't have an explanation for you. I'm just letting you know my experience. Maybe it was a faulty projectile? All I know is that if I'm planning on an elk hunt it is with .30 or .338 calibers I own. As mentioned by a few prior posts ... elk are tough animals. If I happen to be in the same situation with my 6.5 I would try and get closer or go for a neck/head shot based on my prior experience.
Good luck!!!!