Primaries level up from use, unlocking customization options in different slots. Slot availability varies by gun. It looks like all weapons have skins (great for the fashion game). The slots I’ve found so far include:

sights (different sights to taste, or trade off zoom <> ergonomics)

muzzle (recoil patterns mostly)

grip (recoil <> ergonomics/laser/flashlight)

magazine (capacity <> ergonomics/reload speed)

Looks fun! I kinda wish we could slot a bayonet but I can see why they would keep that exclusive