I was one of the very last units to go through USMC boot camp with the M14 including rifle range. We did get to fire 16's on that night fire thing when they allowed visitors and you got to fire tracer rounds. As I was getting out Oct 10, 1973, the new MCRD units were coming in with M16's. I shot 248 out of 250 expert with an off the rack 14 (an H & R). When I went to my unit A battery 1/10 2nd Mar Div, we had M16's. They were POS.
I must admit they made a great hose, but reaching out and touching someone, accurately, not happening. I couldn't hit the broadside of a barn with my 16. I didn't miss with the 14. That is exactly why they ended up bringing them back in the early 2000's as the limitations of the original AR design and 5.56 became apparent. One thing that bugs me is that all the 16's were full auto back in my day. You might be able to carry more rounds for the same weight (prime reason for approving them), but I know most would go through that in a flash spraying and praying. I believe that was why the Marines went to the three round burst version (A2?). One tends to aim more with a 14 and hit what they aim at.
I must admit, that the M4 in urban warfare appears to be the best use of the AR platform in 5.56. Few targets more than 25 yards away and short easy to maneuver in tight places. I note the Marines are probing getting rid of theirs with that new M27 platform. I don't think they have ever loved the AR platform in 5.56 but may have found a core improvement still with the same manual of arms. They are being careful with how they approach it, but you can see that they would like it for all Marines. I think they want it as their primary assault rifle. And it will reach out and touch someone much more effectively is my understanding. Wikipedia has a bit under combat reviews on it. For a failed weapon though the M14 is still around addressing the shortcomings of the AR/5.56 platform that have been there since the beginning. It was a flawed platform (retrofitting the "forward assist assembly" from the gitgo and that "alteration" was nothing more than fixing a fatal flaw of the original design. As you can tell, I do not own any black guns unless you consider my mini 14 one. "Old School"