The Assault Rifle presented in Fallout 4 can sort of makes sense... think of it in these terms, Power Armor was a brand new thing, just 10 years from initial fielding and in that short period there were at least three mass produced lines (the T-45, T-51 and T-60) with several variants of each (A-F). It was a brand new thing that was being heavily experimented with. To go along with that, there was a resurgence of weapon design, weight wasnt nearly as much of an issue anymore. TTPs were still under development. The F4 Assault Rifle may be an early design to make a standard issue rifle for power armor troops, the small caliber would use existing stockpiles of munitions, the water cooled barrel was for continuous use instead of short burst fire. Perhaps the idea was each soldier would be a walking tank laying down huge barrages of small arms fire to suppress enemies. unlike other fighting vehicles that could man multiple weapons at once, a PA trooper would need one weapon that fired for longer periods and had to find a way to deal with all the extra heat that generated. The R91 could have been shuffled around to specific units where Power Armor wasn't fielded in huge numbers such as the capital region. The Combat Rifle was a new issue item for non Power Armor troops who would be using terrain and cover far more, it could be a second line weapon, similar to a PDW in the .45 variant and a DMR/scout rifle in the .308. It would support the suppressing fire of the PA troops assault barrage with targeted higher impact .308 shots. In general everyone was reeling from the way Power Armor changed the battlefield and this was a stumbling first step to adapting to it.
On a side note about the assault rifle in the real world thing... It is entirely legal to own an assault rifle in the US in the real world. It must have been manufactured prior to 1984 and you have to pay a tax stamp for it but that is about it. some states further restrict or regulate them. The scarcity of these pre-84 guns does raise the price up considerably though, the cheapest of them are well over $3,000 with something like an actual M16 or real assault rifle costing at least $16,000. The vast majority of what people are calling assault rifles is a misnomer, the news will also call these "assault weapons" as a nod to the fact that they dont actually meet the definition of an assault rifle. Sometimes they are also called "black rifles" in another attempt to single them out as something other than a semi automatic firearm chambered in an intermediate cartridge. The Fallout universe never seemed to develop the gun control regimes of our world, thus semi-auto rifles tend to only be dedicated hunting weapons where the concept of sport dictates magazine size and firing mechanisms or things in full sized rifle rounds that arent very useful in automatic firing without something like power armor to control recoil.