Just goes to show ya that everyone has their own opinion. I have a 250 Savage Ackley Improved and it shoots about like a .257 Roberts. You asked about "big mule deer" and I personally would not want to shoot a huge 34 inch mule deer at 300 yards with a .250 or .257 Roberts. Not that they won't kill a deer, they will. I just like something that shoots a bit flatter.
I have killed truck loads of deer and antelope with shots to the head and neck. In of elk lost because of failed head or neck fact, neck shots used to be my favorite. Now I only shoot behind the front shoulder. I don't even consider shooting for the head or neck. I have seen several deer and a couple shots. Too many animals get hit in the face, jaws blown off, etc.