Yes the 6.4 X 55 will kill an elk but you are going to be limiting your shots with this caliber, just like I had to do using my 25-06 when I used it to kill a bull elk.
My brother in law years ago used a model 70 Winchester in the .243 caliber and he killed a bunch of elk in his younger days. If the hunter using the caliber can shoot and place the proper weight bullet, all will spell success.
