A good understanding of the answer to that question can be found here: Random Shots: Episodes in the Life of a Weapons Developer
The author worked on both the M14 (T44)/FAL (T48) trials and the M60 programs at Springfield Armory (the actual one, not the Croation and Brazilian importer using the name). IMO, it should be required reading for anyone wanting to come into the small arms field, especially on the engineering side.