[RussEfarmer]

A weapon in Garry's Mod follows the SWEP Structure, which is stored in a file called shared.lua. This file gives information about the single weapon it's attributed to. I'm not exactly sure how restrictive a shared lua file is on changing things clientside, but in order to change the viewmodel for every weapon, you would need to have a way to change the SWEP.ViewModelFOV variable in a lua file for every weapon on the server. I might be wrong, but that's my understanding.

Having a set FOV defined by a slider for every weapon wouldn't be too good anyway, since not all viewmodels are created equal. One weapon could have a viewmodel made to look good at 90, while a lot of other weapons wouldn't look good at all on 90.

If anyone is curious, I've included the default lua file for the Raging Bull in case you want to see what it looks like.