Unless you can consistently clean the NRA MR target at 600yds from your field positions in conditions in which you would take a shot at a game animal, you have no business shooting at an elk at 600.
While I agree with the principle of holding oneself to a specific standard of marksmanship for shooting at game animals, I would point out a couple of things:
For some reason, this notion typically only comes up when we are talking about shooting game animals at 500, 600, etc., yards. I have witnessed the shooting of many hunters who can’t keep all their shots within 12” at 100/200/300 yards from field positions, even in ideal conditions. I have observed hunters who have a far higher miss/wound rate at 200 yards or less than I do at any distance and set of conditions in which I would take a shot (including 600 yards in the right conditions). I find it interesting that we rarely comment here on the forums that people have no business hunting game animals at all until they have practiced and developed sufficient marksmanship to place all shots within 10-12” at a given distance at which they would shoot an animal.
My other point relates to the modification I made to the above quote. There are situations and conditions in which I would absolutely not take a shot at a game animal at 600 yards, and others where I certainly would (and have). This is no different than if the animal were at 300 yards.
It’s fine to be critical of hunters who miss or wound, but are we as critical of a guy who does it at 300 yards as we are of the guy who misses or wounds at 600 yards? A miss or wounding shot is possible for two types of reasons, regardless of distance: factors outside of our control, and factors within our control. I would argue that an ethical hunter should mitigate all factors within their control that might lead to a miss/wound, and only shoot in conditions where the factors outside their control are minimized as much as possible.
