I've killed a poo ton of elk myself and have seen them killed w/lots of different calibers, even suppressed subsonic 22lr. Hit em right, they die as will anything. Some creatures seem to have a tendency to fall over dead without a lot of effort, almost like they know they're a prey animal so just go along w/the program, bunnies are prime examples. Some just have an insane desire to live, like a cat.
From my perspective elk are in the later camp, tenacious beasts that will fight to the end. I hunt Rosie's exclusively and I know there is a difference between them and Rockies. I also hunt coastal timber which is more often than not very rugged terrain. I started out shooting .308, then went to 300 Win Mag and now shoot 338 RUM. My personal opinion, you can't go too big on a mature bull elk, within reason. After slogging 5 miles through the nasties my goal is to anchor that bad boy where he's standing. I'm prepared for the worse case, a monster bull in the timber with maybe only 1 chance to get a shot off before he's gone into a black hole. Gimme some minutes on a broadside bull in a field and I'll put a little pill in his head and call it a day, I've done that off my deck. But if I'm chasing Mr Big out back I've got the RUM in my hands. I've seen too many bones and carcasses in the woods, chased bleeding elk for days, watched elk take hit after hit without going down, killed them w/scarred over bullets and broadheads in them and heard many, many tales of killed, wounded and lost animals from friends, neighbors and locals to ever think about being undergunned. I've was scolded by older locals in the woods for carrying the 308 when I was younger. If you've lived in elk country long enough you'll encounter attitudes, lots of them, don't ever try and tell a local a thing about elk, they'll show you the stack of racks in the barn to prove they're right and your wrong.
Every situation is different but I've learned to prepare for that once in a lifetime opportunity and I'm going to do my darndest to make sure it works out in my favor. I chased a herd for a couple hours one morning in the timber, finally got onto them w/out them knowing I was around. They started coming up a hill single file at about 50 yds in front of my. One stopped with it's head behind a bush but I could see the antlers above it. I waited, he finally took a couple steps and stopped broadside at 50 yds between trees. I put a 200 gn through the lungs/heart w/my 300 win mag BAR, saw the hole, he turned and ran about 10 steps and stopped and looked back at me. I put another hole in him about 2" away from the 1st, he turned and ran back down that steep, nasty hillside and died 75 yds away wedged into some deadfall. That was a day. If he would have died where he was standing it would have saved us about 3-4 hours work.
I've taken 2 6x6 dark timber bulls, the last one I had about 2 seconds to make the shot at him quartering away at about 70 yds through the timber. He kept going but dropped w/in another 50, I'll keep shooting the RUM until I can't.
Those old timers used what they had and did kill a lot of elk but they didn't kill every one they put a hole in, I'll pretty much guarantee you that.