Unless your marksmanship skills far surpass those of mere mortals like me, a nice hot 180-ish grain 30-06 load will well and thoroughly kill an elk (or moose, or anything else on this continent) at any range you have any business shooting at one. 300-ish yards would be well within the accepted energy/velocity recommendations, and if you think you can do 400 yards under field conditions, fling a Hornady LightMag in the tube and let er fly. (though, personally, I think even 200 yards is a LONG ways under field conditions...)