First of all there is nothing the good ol' .308 can't do to 500yds.in the 48. (some may argue this)
To what range do you plan on shooting?
What game will you be shooting mostly?
How heavy of a rifle will you put up with?
Are you thinking of light weight? 7lbs. or less or something in the 10-15lb range.
From reading your post what came to my mind was a 7MMWSM
set up to shoot bullets in the 160gr. - 180gr. range,built something maybe like this.....
this rifle weighs around 12lb with optics.