Jones RE said:

I read the author as recommending a weapon with better long range capability and teaching soldiers to use more than the most basic fundamentals. I can't think of a sufficient reason not to do both immediately.
I think that US military knows how to train sharpshooters. To improve situation this means that every soldier must pass Squad Sharpshooter program. This adds 1 week to training if I understand correctly.
For a long time there was available "Squad Sharpshooter Concept" in internet by Michael R Harris http://www.dtic.mil/ndia/smallarms/Harris.pdf It has disappeared now

About ammo and calibre. For some period I used very often Soviet ammo 5,45x39 (brain child of Soviet engineers that figured out that US new M-16 is "better" than AK-47) and 7,62x39. You can make just one test to compare the effectiveness. Arrange night shooting with tracers on the filed where grass is above the waist. With 5,45x39 you can see nice vertical rocket show in the sky with few holes. With 7,62 the picture is much more horizontal. I presume that you can see the same picture if you test 5,56x45 vs 6,5/6,8.