It is said that to reliably, humanly kill an elk, all things being equal (shot placement, bullet characteristics, yadda yadda yadda), something delivering around 1,800 foot pounds of energy would do. The OP mentioned 400 yards maximum range. So get a caliber of rifle that will deliver 1,800 foot pounds of energy at 400 yards. And then be accurate with it.

Not a whole lot more complicated than that.

Last edited by flagstaff; 04/02/22.
"Successful is leaving something in better shape than you inherited it in. Keep that in mind, son." Dad