I have killed elk with 308s and you need not worry about it "running out of power". That's the tripe of the marketeers trying to sell products, but ignoring the truth and the real world.
However you are WAY better off not setting your rifle up for "long shots" because you'll find they are rare compared to the shot you'll get at between 100 and 300 yards.
I use 165 grain Nosler Partitions.
They are very accurate, open up well at long ranges (longer than I like to shoot) and hold together at normal ranges far better then most "long range bullets".
If you are going to use the AR10 style rifle I would set it up to use the Noslers and learn what it likes. You can always carry one short magazine in your pocket with the ELD-X bullets in case you see an elk "way out there" and for some reason you can't get closer. It's very fast to change ammo in an AR.
Me......... I don't. I shoot the flat base Noslers to any range I can shoot. But that's just how I do it. I don't say it's the only way.
I have used them from my 270s at VERY long ranges, but I don't any more. I see no real need to do it, and lots of reasons not to. So far I have been lucky, and I have never fired a shot at a game animal at loooong range that I have not killed, but I think I have pushed my luck as far as I care to.
So I just don't shoot past 500 any more.
I have not had a reason to ever fire at an elk over 400 yards in my years of hunting and guiding for them . Not ever. In fact, in all my years of hunting elk I personally have had to shoot past 200 yards 4 times. And I have have been doing this now for about 42 years. So when I speak of this it's not theory. It's based in 4 decades of experience.
Have no fears at all about the 308 cartridge and elk. Just use a tough bullet and you'll be just fine.
While I have no doubt that this is true for you and your location, but it hardly holds true throughout elk country.
In the last dozen+ elk, I have shot exactly 1 under 200 yards. The rest have been between 350-450 yards. Most have been with a .308, and not one needed a second shot.
That said, due to the open terrain where I hunt, I often use a .300WM, in case of longer shots/windy conditions. However, if I knew that my shots were all going to be under 400 yards, I would not bother with a magnum cartridge, and be totally content with a .308.
