I've been hunting elk in Colorado since the late 90's. I started with a .270 pushing 150 NP, moved up to a .300 WM and Barnes 168 TSX loads three or four years later. I've killed a lot of elk with both. I've always taken a back up rifle just in case, and so far have never had a need to fire a shot from the back up. Most years it was a 30-30 Marlin. This year I have a new FN .300 WSM
for my main rifle, and will be taking my .243 for the first time as a short range solution for the timber, or back up if need be.
I think it all boils down to being honest with one's self as far as their abilities and experience merit. I've put a good number of the deer and elk down with a neck or head shot, and I know exactly what my limits are for all of my rifles. My .300's can certainly reach out there, I know I can place bullets on paper at extended ranges, but I am not good with trying to end a game animal with those loads past 650 yards. My .243 is limited to elk at 200 yards, and only on a stationary target, calm conditions and a very calculated send.
I lost a cow two years ago on a 700 yard shot from my 300 WM that I saw hit hard. We tracked her for over a mile, saw that she bedded four different times, but she continued to chase the herd on to private property that we couldn't get permission to recover her from.
It's all about knowing the limits of your gear and yourself.