Actually you are wrong on just about all counts.
There is a background to the design of a GUN ...and ques what? It was not ever design to be a shoulder fired weapon.
Sorry, but that's just an objective truth. Your point? Things which are not designed hand held can not became hand held through development? Really???
Objective truth is that split occurred sometime between 1945 and 1961 (exact date is not specified by lore). M16 came to service in 1962. M14 is not assault rifle. AK-47 is Russian weapon and newer was in US service. It did not became available in USA before 1981 ...and then only as Finish AK copy Valmet M-62S.
You're wrong plain and simple.
Assault rifle is not battle rifle. Not even in Fallout alternative universe.
Everything comes at the price, air cooling included. But whatever the price was, it was good enough for mass produced weapons used in war.
Rifle was also newer chambered for laser beam. Again, comparing to our timeline is completely irrelevant. That something happened in our timeline does not men it happened in Fallout one as well and vice versa. That applies to question of efectivity as well. That we made some design choice that proved to be more effective does not mean that people in Fallout universe did that as well. They still are using vacuum tubes happily even if microprocessors are more effective. On the other hand they designed micro fusion battery powered laser rifle while we still use primitive assault rifles in 5.56. how about that?
Hey, you dislike the FO4 assault rifle? Good for you. You think it looks crap, more power to you. Just don't pretend its not logical or somehow less efficient. It would make a good assault rifle for your average grunt who would have used power armour to shoulder it, nuclear powered robots to maintain it, and nuclear powered cars to bring enough ammo to feed it. Power armor might have being more rare by pre war standards but so were assault rifles who were used to equip only units with power armour. That it fires 5.56 is appropriate for assault rifle as I said, when you add liquid cooling of barrel rifle will weight more then air cooled, but it will compensate for increased weight and more difficult maintenance with increased firerate and durability of the barrel. The FO4 "assault" rifle is progressive and justly included, not mentioning it's aesthetics of using it with power armor. That is bonus reason.
You are fully welcome to continue banging your head on the wall, but you are convincing no one of it being anything other than your personal preference. Mechanically and historically, it's proven and mass produced, battle tested design. Lugging around a 14 lb POS, when I have suit of power armour which can easily handle miniguns is well worth increased firepower and durability of weapon.
Had they merely labeled it a "machine gun" and chambered it in something like .308, that would have been fine too. But they did not, they have given us a beautifully designed assault rifle which fits in to Fallout universe more then copy of M4 ever would and you will live with that. You have answers to your answers. If you choose to ignore them, then you are basing your position on something other than facts.