One thing I have found over several years of hunting is that no matter what you try, no matter what you buy - carrying a rifle just isn't that comfortable.  True a lightweight rig can save your back a bit, and a padded sling may ease the shoulder strain some - but you still need to move that rifle around and try it in different carries to be able to stand it for any length of time.  The following article from Outdoor Life magazine details the pros and cons of the three main sling carries.

I have adapted my own style - it's kind of a mix between the European Carry and the African Carry.  I keep the rifle pointed barrel ahead, but perpendicular to my shoulder.  With a hand on the fore-end of the stock, I can balance the weight of the rig, manoeuvre around trees, bushes and rocks - and with a quick lift and half turn be in a shooting position that wraps the sling around my wrist, which I find helps to lock the rifle in the shooting position and help to steady the shot.