Shoulder slings will not be comfy with a heavy rifle and you will constantly fight it.
For hands free hiking:
Try a 3-point sling, and sling it away from your legs around your side or back for hiking. Works even with a backpack/frame pack.
If you're wearing a real backpack, try a Kifaru Gun Bearer. They work pretty well.
Or, most comfy for long treks, strap your rifle to your pack, most packs have side pouches for water bottles that fit the butt stock perfectly, then use compression straps to hold the upper rifle steady.