With my 338 wm i am shooting 1" groups @ 100 yds using federal trophy copper bullets. only problem is i got zero expansion as far as i could tell on two pass thrus on my elk this year-entry and exit holes appeared to be the same size. shooting core lokts i get 2" groups. typically 2 bullets real close to each other and one off a bit. is the expansion of the lead bullet in the core lokt worth the loss in accuracy when looking at 400 yd shots across canyons?

2 inch group size would not be a problem when you can get better expansion out of the core lokts. 2" MOA is more than adequate for a sporting rifle on elk at that yardage. Everyone likes to see 1" groups but 2 will do. You might try some other makes of bullets and may find a sweet one that expands and has 1" moa

There are a great deal of premium loaded ammo from Federal, Nosler, Winchester, Hornady and the like loaded with premium bullets (Nosler, Hornady, Speer, Barnes) that in my opinion will likely produce better groups than 2 inches @ 100 yards and will do it with a bullet that will outperform core-lokts. I prefer Nosler AccuBonds, Partitions and Ballistic Tips myself and several companies load with their bullets...but Speer, Hornady, Barnes and many others make high quality hunting bullets that will far outperform the Core Lokts both in accuracy and performance on animals.

Remember, a 2 inch group @ 100 yards opens up to at least an 8 inch group at 400 yards (at a bare minimum).

...........and that's from a bench !!!

I wouldn't feel comfortable hunting at 400 yards with a setup that was giving me 2" groups at 100 yards from a bench. I would make changes until I got MOA starting with trying different Ammo.

You didn't mention the gun type, bullet weights or anything. Just assuming you would be going with 250 gr in both types of ammo. You could try hand lapping the barrel and making sure the barrel was free floated except for a pressure point a few inches from the end of the stock if a bolt action. This will often settle down harmonics. You can use anything from a business card folded up to a cut down piece of navcon. Ultimately what would be beneficial would be to attach whatever medium you decide on to the stock so it doesn't shift if you disassemble the action from the stock. It will not specifically improve the coreloks but you can often shrink groups in general. I wouldn't want to hunt with a 2" gun at 400 myself. To many variables out in the field and that's just adding to it. You should also know exactly where your first cold bore shot will strike and if a fouled bore is needed to make it more acceptable.
Good luck.

Lets see. Hole in hole out dead elk. What kind of damage between the holes? Did you have to track a long way? Was there a blood trail? Why do you think the bullet failed? The last two elk I have seen shot, cows, with Barnes TSX bullets small hole in small hole out lots of lung damage and heart damage and both fell within 25 yards. I guess I need more info than two small holes in a dead elk to say failure.

i didn't exam the lungs but when i got him back to camp is when i noticed what appeared to be a lack of bullet expansion. actually the first shot was a bad shot and broke a let @ 400 yds. he moved off up the mountain and laid down @ 480 yds and i was able to get a good rest and finish him. the first bullet @ that range i could tell hit him and the second one his head went down so we proceeded with the recovery. the outfitter i was hunting with indicated he has had issues with the copper bullets when his customers used them. he was addiment that you should never use them. just his opinion but he sees more elk shot each year than i will in my life time. i have had pretty good luck with my groups shooting nosler 210 partitions but i can't find them now. i would be more than happy to shoot those if i could find them. i did find some @ cheaper than dirt but they were a little different load with slower velocity. surely someone will have them again before next season.

As far as accuracy goes, I would try to find a better round/load to get what you are talking about for the longer distances. As far as no expansion, I hear what your saying, but in reality, you can't base it on what that the exit hole is somewhat larger than the entrance.
You get what is termed as the tear factor upon the round traveling through a media, here it is elk tissue, and the kenetic energy is transfered through the round, destroying everything in a funnel shape pattern. Projectile mushrooms as designed, stays intact, exits. Vitals area and surrounding tissue are destroyed, massive blood loss and shock insue.
I just shot a whitetail buck this past Il gun season facing me with a .460 pistol shooting a 225grn Flex Tip at 60yds. The round destroyed 5" of brisket, turned the lungs into water, and blew 4 ribs apart diagonally upon exit, leaving only a hole the size of the tip of my thumb. Buck died in its tracks before his knees buckeled.

All the more reason to roll your own, get the bullet you want and tweak it to shoot as good as your rifle will
RR