For a little fun I decided to do a more retro take on the Assault Carbine and Marksman Carbine in Fallout: New Vegas (no AI, I used GIMP and Adobe Illustrator). Really my gripe is mostly with the Marksman Carbine because it has a lot of GWOT-era late 2000s features, like the ACOG (adopted by USSOCOM in the 1990s), a precision adjustable stock, and quad rail that extends past the gas block. I know having contemporary weapons as of the game's release isn't new to Fallout, it was quite common in the classic Fallouts, but I think it just looks very out of place in the 3D Fallouts which went for a more retrofuture and less Mad Max aesthetic. That's just my opinion though, I think plenty of other interpretations could work. I'm just most loyal to the Fallout 3/New Vegas generation.

My changes were:

1. Redesign the New Vegas Service Rifle receiver so the side mounted, reciprocating charging handle can function. Although this makes the forward assist redundant, I choose to retain it for aesthetic reasons. As the AR-15/M16 procurement process was less than optimal in real-life, I could imagine a pre-war government inserting the unnecessary requirement for a forward assist so soldiers wouldn't attempt to smack the charging handle into battery and possibly damage it. The charging handle isn't monolithic to the bolt carrier, it's a screwed in piece, so I could perhaps see it being fragile. Plus the forward assist is actually seen in the game being used correctly when it jams from being in poor condition, and didn't want to waste the game devs actually getting that part right.
2. I gave it an original Colt Model 607 sliding stock, but in wood as the NCR in New Vegas have Service Rifles all in wood and it likely not be possible to source raw plastic. This design, especially with the wood, might be fragile and an aluminium CAR-15 stock could also be an option, but I'm really going with the vibes on this one. Similarly, the handguard is wood and also Model 607 length.
3. The muzzle device is the moderator from the XM177E1.