I don't care what caliber you use if you hit an elk or deer for that matter wrong what caliber you use is about the last of my worries. For people shooting whatever the yardage is the caliber has to have enough poop to get the job done on less than ideal angles. Bullet selection does play a part. My favorite elk caliber is a 270 Winchester using 150 grain bullets we have 3 of them and all have taken multiples of elk thru the years. Sometimes I'll use a 300 wby and we use other calibers, I have used my 257 Wby on elk and personally it's a little light in the bullet weight for me to classify as a one gun rifle for elk and tougher animals. You can easily get by with a 308 win at 400 yds on any elk as long as you hit it right, one of my favorite mtn rifles is a Kimber 84m in 308 with a Leupold 3x9 ultralight scope and the rifle tops out at exactly 6 pounds with scope , mounts, sling and 5 rounds of ammo. That 10 pound 300 Wby with big scope is getting left behind more and more. As long as you have a solid rest in a known reliable/ trusted rifle animals at 400 yds is very doable with practice. My wife is using a little Vanguard in 6.5 creedmoor on her up coming trophy deer hunt on the Utah border in a couple weeks, she's killed elk and antelope with it. It's actually becoming one of my favorite calibers , it originally had a little Nikon 4x12 bdc scope on it and it broke and sent it off for repair, so I had Zeiss 3x9 plex reticle laying around and put that one in an emergency, her ranges are going to be substantially reduced to about 300 yards or so mainly because she's not use to the scope, but soon I'm going to put that new Vortex Diamondback 4x16 front focal plane on it for me to shoot clicking up longer ranges. I'll be taking a couple spares her 270 win and my 300 wby only because I have a lion tag in case I see one. I got her a couple years ago that Caldwell DeadShot Field-Pod the short version and bought a little packable lightweight shooting stool , this tripod/field-pod with rifle supported both front and back is pretty darn solid except in heavy winds and that's iffy at best anyway. This set up get's you off the ground and clear of ground vegetation.
For the person that can only afford one rifle and even though I firmly believe beware the man with one gun because he knows how to use it is pretty true in my opinion, BUT really how many of us out there have only one rifle that have been hunting for awhile. I have current friends and had friends no longer with us that only have one rifle for pretty much their whole life one in particular who is now gone mainly used a beat up Jap Arisaka chambered in 6.5 x 257 Roberts and was an elk killing machine when he lived in West Yellowstone and then in Arizona, he finally bought big box store scope combo Savage in 30-06 before he died thinking it would be an upgrade, wrong he had nothing but problems with that rifle. Personally I think it's huge mistake to go hunting any distance from home without your own spare rifle or use of a friends spare rifle, PERIOD !!! It happened just a couple days ago a friend had scope problems and blew a shot on a big bull elk , hit it and lost it, and this guy is no stranger to guiding for trophy bull elk, so the best hunters still make mistakes, granted he wasn't hunting but 2 hrs from home that screw up blew a whole day of hunting. Another buddy had a better known scope freeze up on him, and then it broke totally in eastern Oregon on an elk hunt in the 80's, I loaned him my rifle and my dad loaned me his 270 win, he enjoyed being the camp cook the last years of his life and not in the best of health. I froze the 3 position safety on dad's rifle until it unfroze and I shot an elk with it. My son I guess 3 years ago took a fall on a Coues deer hunt and totaled his scope and we had to drive about 60 miles to the nearest gun store and bought a little Vortex Diamond Back to get him by. I've had the trigger screw up on a Weatherby in the field and had to take the trigger apart clean and put fingernail polish on the one trigger housing screw, finally I glued the darn thing in there so until I get a Timney to replace it might be a little bugger getting the old trigger out. I cannot tell you how many times I've seen guns in general screw up in the field either other hunters, friends or family. For 300 bucks go buy a used rifle with scope and keep it as a back up. You don't go in the field without a spare tire or 2 so why go in the field with one gun especially hours from home.
If I had to absolutely choose one rifle I would choose a carbon fiber barrel with a min of a 24 inch barrel add a muzzle brake, rifle weight fully loaded with scope, ammo ,sling, mounts no more than 8 pounds, most likely a Leupold VX5-HD 3x15 with CDS dial in 300 win mag caliber. Why 300 win ? It's totally accurate, proven, it will take anything in North America, and African Plains game, it can be loaded hot with 220 grainers and down-loaded for reduced recoil sensitive shooters and ammo is available almost everywhere.