I had a quick google and apparently that picture is of a LRRP unit in the central highlands. Pretty interesting actually - most likely (considering the role and the area) to have been a member of the 1st Air Cav that were moved into the LRRP role at the time who coincidentally were one of the first units to get the M16. By "tapered grip" do you mean the foregrip? As I've personally only ever seen 60's/70's era ones with the tapered, triangular style. There is suggestion that civilian spec AR15s were supplied to SF units (presumably to try and keep up with supply requirements) so it's perfectly possible I just couldn't make out enough detail in the picture.