Dr. McD said:
Because modern weapons have been in series since the first game, the modern era still happened, it was very different from the real world but it still happened, the world of Fallout didn't just jump from 1950 to 2077.
It's not hard to see why they don't fit Fallout 4's world, though. Kinda seems silly to turn Fallout's weapon roster into something painfully TACTICAL!, like assets have been accidentally mixed in with a new Medal Of Honour or CoD...
Everyone's load order is their own, of course, so if someone wants to CoD-ify Fallout, fair enough. I dislike all the SCAR's and wotnot, but I do use the P99, which certainly doesn't overly fit the world if you look at some of the other pistols in the game and series. I don't use the rails or go nuts with attachments, however.
My favourite weapons have been the Mauser pack, the M16 (probably my most used AR in the game), the Remington 1858 (my go-to hand cannon) and the 1911, though for general use the P99's more or less my default side-arm since adding it (it's well balanced for damage, has a good rate of fire, and just handles very well. good for picking off some lower level targets, and also good in a bind when you need to just unload whilst backing off). Custom reload animations are nice, but I try to avoid them like the plague given I use Alert Carry, and custom reloads don't conform to it.
The only weapon I keep now (I love the M1 Garand, but got sick of the perma-aim. I've not got rid of it, as Nick in the black modded trenchcoat looks rather badass using it) which does have a custom is the IF-54, primarily for a more dedicated rifleman/sniper character. It doesn't really fit the world, but it does suit her more hi-end, stealthy build and look, and its sniper configs can hit hard.
Re the Fusion gun: huh, hadn't heard of that before. I might give it a try whenever I go back to Fallout, as it's also on Xbox.