Armalite and Artillerie Inrichtingen (AI) experimented with a number of different belt-fed machine guns / auto-rifle versions of the AR-10. The photo above and below are of different prototypes. In the below photo the rifle is fed from a 250 round belt fed from a backpack. This concept was invented decades before the ammunition belt backpack which was featured in the movie Aliens (1986).
None of the AR-10 LMG prototypes functioned well and it never went into production.
Thanks to Chris for sending in the photo.