A straight 4X was all we used for years for all our big game hunting rifles, from 7mm's to .375's. I ran a 2X on my .458 American.

I shot the heads off 34 grouse one fall with that rifle while out moose hunting. For grouse, I was using 230gr. Speer .452" pistol bullets and 18gr. of AL8 powder. Those undersized bullets slugged up and hit right on the lower post of the reticle at 25 yards, which is the range most grouse are shot at, around here. They ran 1,350fps and would make 3" groups at 100 yards, with care, would easily kill deer and black bears out to that range as they were about the same power (little more, actually) as full powered loads in my 4" .44 Mag.

Today, the 4X Lyman All American Perma Center that I have mounted on my .30/06 is all the scope that will be hunting with me this fall.

165gr. SST at 2,945fps will certainly handle my elk and whitetail deer out to 400yards, as well as a moose if I happen across a young one. The 180gr. SST's at 2,755fps strike only 1" lower at 100 meters.

I full expect to not have to shoot past 100yards, on any of them, except perhaps the deer.

A 3-9x40 is more scope magnification than needed for ANY .338Win. Mag.

A more powerful scope will not help you to kill animals at extended ranges (you should not be shooting them at - imho).

Practice with what you have at ranges to 300ayrds, then practice some more from hunting positions - prone, kneeling, sitting and standing - all with AND without resting on objects, like trees, fences, bushes, etc.