I killed my first bull with a .30-06 with a factory made round. If was a Frontier 150 grain bullet (I don't know if they even still exsist) and the elk was standing broadside at 170 yards. He took one step and fell over. My next couple of hunts went bad and I retired that 06 and bought a 7mm Remington mag. With that rifle I have taken over 20 elk shooting various 150 grain bullets with ranges anywhere from 10 yards to 400+. The only one that I had go anywhere was a cow and she made it well over 100 yards before she figured out that she was dead. Most of them would either stand there and wonder what happened or just take a couple of steps and fall over. The last few years I have been using a .340 Weatherby Mag on elk shooting hand loaded 225 gr Barnes X bullets or 185 grain Barnes XLC bullets with the furthest shot being at 756 yards per the range finder. That was a 7x6 bull and he took two of the 225 gr X bullets through the boiler room before he dove into the oak brush, the first shot spun him around and the second drooped him. The first shot was all that was needed but my spotter wasn't talking to me so I sent another one downrange to the elk.
Now for the most part my good old .30-06 would of done the job on most of the elk that I have shot except for that 7x6 bull. I would of kept on using it except that you knew where the first shot was going but if you needed a second shot it was getting iffy. That rifle was bought new by my uncle in 1935 and I couldn't tell you how many animals that he shot with it before he gave it to me when I was the ripe old age of 5. I do know that I shot more than a 1000 rounds through it before I retired it to the gun safe. The big reason that I went up in caliber to the .340 is that I read about it when it was a wildcat and loved what the balistics were and since that time I always wanted one.