A lot of good comments here but seems it went off track a little from the original OP's question.
Large Elk hunting at 400 yards or more is real wilderness hunting in Canada or in the Western USA, in my experience you can spend a week hunting in this type of environment and are very lucky to get one good opportunity at a suitable animal. The OP is smart to think that some of the shot opportunities could be 400 yard shots.
The OP asks if a 6.5 is a good choice of caliber to carry into this scenario. I commented earlier that I would select something bigger and that it would be a magnum caliber. I agree that a 6.5 can kill anything walking on the planet at 400 yards. However a good size Elk can be over 700 lbs or 300 kg, shooting at the range the OP describes, I would be much more comfortable with something larger than a 6.5.
Most hunters in the US and Canada are not considering suppressors for wilderness hunting and I would say that few have muzzle breaks. If you are firing one shot in a week of hunting the last thing you are worrying about is the noise or blast. As far as gun weight most magnums are similar in action to a .30-06 and may have a few more inches on the barrel, the weight difference is not that large, it's also a question of what else are you carrying on a remote wilderness hunt. You will not even feel the recoil when you fire that one shot that you have been waiting for all week.
I personally hunt North American big game, African plains game and African dangerous game with a range of larger calibers that I own, these include 7mm Rem Mag, 300 Win Mag, 338 Win Mag, 9.3 x 62, 375 H&H and 500/416. In the UK I tend to limit my deer shooting to a .308 these days although I still hold a .338 WM on my ticket. It is certainly true to say that larger calibers have more recoil but that is something a shooter has to manage, I have shot a serious amount of long distance rifle from 300-1200 meters, I would argue that once you get to 400 yards and beyond the magnum rifle is actually going to be more accurate than a smaller caliber. Not too many military snipers shoot small calibers.
The key reason I would not be carrying a 6.5 would be my concern of wounding an animal, and not recovering it. The second reason for not carrying a 6.5 on a wilderness hunt is that dangerous animals such as brown / grizzly bear live in the exact territory that Elk live in, I want something heavier than a 6.5 to shoot a brown bear that is keen on eating me, my third and final reason for not carrying a 6.5 is that if you are in wilderness territory and may anticipate a 400 yard shot it may also be a possibility that the only shot you get is actually a 550 yard shot. I'm not supporting all these long distance shots but I'm saying you better know your rifle, how to shoot it well and how to shoot accurately over long distances if you are going on this type of hunt.
Here is what Wikipedia has to say about the sporting use of .338 WM:
"In North America, the .338 Winchester Magnum is most commonly used for the hunting of larger deer species such as
elk and
moose. It is quite popular with elk hunters, with bullets ranging between 200–225 gr (13.0–14.6 g) generally preferred for large class 3 game such as elk or moose.
[10]
Apart from the larger deer species, the .338 Winchester Magnum is often used for the hunting of and defense against dangerous class 3 game, particularly the great bears including grizzly, polar and brown bears. It is often carried by fishermen, hunters and guides in Alaska and Canada for protection as encounters with these larger bear species can be common.
[11][12]
The .338 Winchester Magnum can be considered a good all-round plains game hunting rifle in Africa. It has also been found to be effective against the big cats where hunting allows for the use of the cartridge.[
citation needed]"
I shoot a lot of deer, I hunt UK, Europe, North America and Africa. I've shot a lot of animals from Eland down with a .338 WM at all sorts of distances, my personal view is you need to take something larger than a 6.5 to hunt large Elk at 400 yards in the Canadian wilderness. If you invite me I will be showing up with a minimum of .300 WM or my likely a .338 WM.
Good luck on your hunt.
