I have used the 6mm for bison culls at short range with behind the ear shots at it's shattered the spine with instant kills.
I haven't met an elk yet that would sit there like a bison on a ranch and allow you to shoot them behind the ear.
Why on earth would you want to take a 6mm on an elk hunt, especially sinche you have other rifles? Sure it will work with perfect shot placement but you had make darn sure that bullet is going right where it needs to go.
I think a .270 is a pretty decent elk cartridge with the right bullets and i might even use a 25-06,but a 6mm is
really pushing it IMHO no matter what bullet you use.