def4pos8 has it right. Both rifles were equally acceptable, the M14 was a home grown design and it was a product improved Garand. There was nothing really wrong with the M14, and as the Garand had proved itself every where, and the M14 was similiar, why change?
I have dealt with the "user" community. What the military user wants is something similiar to, but better than what they have. They are conservative by nature, once they like and are familiar with something, they don't want want radical changes. So they tend to be evolutionary not revolutionary. And if you are going to put your life on a piece of equipment, you might tend to buy the stuff that you know is reliable. Of course if the revolutionary item really is a big jump, lets say Death ray guns versus pointy sticks, the user will happily put the pointy stick behind the firing line. He won’t junk the stick until the Death ray proves itself 100% reliable..
The basic problem with the M14 versus FN comparison is that strong points in one platform is not found in the other. So it becomes a choice, of what do you like. Apples or Oranges.
We might have been better off adopting the FN. As good a job the Ordnance Department did in promoting its in house design against the FN, they could not hold the line against Colt. Colt hired lobbyists, besmirched the M14, lobbied Generals, Congressional Staff, and Congressmen, with Broads, Booze, and Beefsteak. The Ordnance Department is totally outgunned in the political area by Commercial contractors. There is no line item in Government Contracts for lobbying, pay off's and corruption. So, in the end we got the M16 through a top down decision from the Office of Secretary of Defense.
The FN might have been able to survive the lies of the Colt lobbyists, and we would still have an effective rifle in inventory.
If you notice, DoD really does not have any inhouse design bureaus. The era is over when DoD produced inhouse designs that took profits from contractors. DoD just puts “requirements” out on the street and has a competition between contractors. Since all contractors fully fund Politicians, snatching a program from Defense Contractor A and giving it to Defense Contractor B turns out to be a fight between Congressional interests. Closest thing to a fair fight in the Acquisition process.