What you probably want is some variant of the M1907 shooting sling, which you can brace your arm with while prone.
http://thecmp.org/training-tech/armo...-installation/
To carry a long way with a heavy rifle you should have a biathlon variant of it, to center and spread the weight over two shoulders:
http://www.eurooptic.com/tab-biathlo...ups-black.aspx