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
I haven't had a chance to play with it yet, and with the unlock system it might take a while to really get to see what it's capable of.
That being said, this also have us more visibility on the weapon stats so we might be able to get a sense of scale by looking at the values of familiar weapons. For ergonomics, the dominator has 25, the penetrator has 43, regular liberator has 57, and the defender has 85.
One attachment picked for ergonomics gives you about 10, so almost the difference between liberator types. A penetrator with every attachment optimized for ergonomics is +36, which being more than the difference between it and the dominator and almost enough to handle like an smg (at least until you start firing lol) is pretty significant.