I don't think anyone has any business shooting that far at big game in most cases. I here all of the arguements about practice, etc, butthere are a couple other factors to consider. One is that at that distance a animal can move right as you pull the trigger throwing your shot right where you intended, but he animal has changed position and is now a wounded animal. A target at the 600 yard range doesn't move right when you shoot.The wounded animal is the other thing to consider. Even with perfect shot placement, a elk can go a long ways even with a 250grain bullet out of one of the hot 338s. By the time you walk 500 yards in much of elk country, it is hard to find the blood trail and even the animal, even when you think you knew exactly where it went. On flat open ground, that is probably fine, but much of these shots are going to be cross canyon shots making the time it takes to get to the animal take a long time.
All that considered, I have my eye on a weatherby accumark in 338/378 just in case I succomb to the temptation to take large game at longer than average ranges. Any of the big 300s or 338s should have enough energy and in a good rifle they are all accurate enough.
My longest shot was at a coues deer last year at 330 yrds with a 30-06. Hitting an elk at 500 would probably be about the same as hitting a tiny coues at that distance. The deer dropped at the shot, but it took me a long time to get to the animal and if it would have moved much at all, recovery would have been very difficult. It was my only shot on the last day of the hunt, so I took it.