Your question is beyond a blanket answer. I was Army Infantry in II Corps, the central highlands. In that area most of what was contested was rice patty and more flattened terrain. In first Corps, and over toward Cambodia, the overgrowth was especially thick, and visibility sometimes less than 3-4 meters, so to return fire in that area was totally unrelated to responding across a rice patty dyke where we often had visible opposing forces. I was never in the Mekong Delta, but from being in country I know infantry operations were limited. The US Navy undertook a large part of the clearing operations from RAG boats (River Assault Group). I mentioned all this to try and point out that the terrain dictated the response. [One must remember that no ground soldier wants to waste his ammunition - because there is NO assurance of resupply on a timely basis - for instance, command elements might not even know where you are, much less how to get to you.]
Thinking about resupply leads me to answer a question you didn't ask. Unfortunately we have to be realistic and understand that one of the most prevalent sources of resupply is what we call "battlefield pickup." This is tough, but its realistic; when someone has no further need for his ammunition, or weapon - it has to be picked up and redistributed to those who can still fight. [Hopefully you are beginning to see that fast triggers that spray bullets are counterproductive to success.] To take this a step further, if we are thrust into a fertilizer/fan situation, what caliber of ammunition do you want your weapon to use? The one most readily available laying around. With that in mind then the .223/5.56 is the clear winner - everyone's got it. Military, police, preppers, you, me, and on and on. So why would you build or buy a weapon outside those parameters? That answers your second question. The AK has advantages, but the 5.56 relies on tumble, and disintegration on impact to get the job done. It does not always get the job done on first hit, but I promise you it will slow the target enough to get a second shot.
I am nearly 71 years old, and what I just shared with you, I learned when I was 22 years old. Hopefully that gives some perspective on combat operations - and the fact that the terrain characteristics dictate the type of outgoing fire. Someday maybe I'll share with you the story of how a 22 year old kid learned it's not smart to load his magazines with all tracers. Tracers are visible from both sides, ya know.
