210 gr Nosler Partition has always worked for one shot kills for me. I don't reload, so always been factory ammo. My A-Bolt and X-Bolt have both shot this bullet well. I have 400 rounds of Hornady Superformance 200 gr SST ammo that shoots well, but I can't draw a rifle tag to try it on anything but paper.
I have a new WInchester Model 70 weather extrema and my friend shoots an A-bolt. We use 225 gr. nosler Accubond with 72 grains of Reloader 19. At a 100 yards my bullet holes will touch each other. I recommend changing the butt pad to a softer one to absorb the recoil. A muzzle break would help with the recoil as well. I shoot from the prone position, as this helps with the recoil and how I personally shoot more accurately at the range. I wouldn't use a lead sled because the the lack of recoil could break the reticle in the scope.
I shoot Federal Fusion .338 Win. Magnum 225 Grain off the shelf ammo. Have had zero issues getting my Win Mod 70 to shoot well with this and for critters to die fast. A few shots beyond 300 yards but most in the 200 yard range. Elk, mule deer and mountain goat.
That's one of the best things about a .338
Just about anything you can shoot out of it will be very effective on elk.
I don’t have time for reloading, so I tried a number of different factory loads.
Mine shoots 210 Barns Vortex in to 1 inch circles at 200 yards.
Can't tell you what to use but stay away from hornady 225 spitzers. I shot rag horn n a cow with mine and while both were one shot kills neither bullet hit the far ribs. Bull hit the stomach which is a hellova stopper but with that big of a bullet going that speed I thought I would get better performance. I'm playing with 250 nosler partitions for next time