I think its worth noting that modern guns are mostly polymer and plastic, and in a oil scarcity scenario, making your guns out of things whose main materials are oil-based, is not a good idea. Then again, there are non-oil based ones, but I suspect these would go to more important things, like making combat armor.
The R91 is a good CETME-based design, far more original than somesort of plastic AR-analogue, I would say.
Personally, I like the metal + wood furnitude look of Fallout guns. I think it really stands out a lot from generic sci-fi guns found in something like Halo or Doom, which are all about shiny plastic and space-age metals affairs with nice shiny LED ammo counters and gadgets and whatnot. Even some Fallout energy weapons have wood grips or stocks and whatnot, giving them a super-retro look. The techy stuff usually has chrome.
The real problem is that Bethesda can't into designing guns. Meanwhile Fallout New Vegas is gun pornhub.
Don't like all the New Games designs, tho. Combat Shotgun from FO3 would have been a good design (I mean, its literally a Ppsh) if the drum wasn't so far forwards. Also I really dislike the designs of the AER9 and the AEP7, they look too blocky to me, at least compared to the Watzz Laser Pistols and Rifles from 1/2/FOT. More of a 2000s design with retro sensibilities.
What's the most "modern" looking gun of the new games, anyway? I would say the Assault Carbine and Marksman Carbine. Always found them somewhat innapropriate in-universe. Then again, seems like they were specialist equipment, not some mass-manufactured rifle given to grunts.