When you attach a item that has its own visable attachments to a vehicle the attachments arnt visable.
I made a weapon slot on my modded car. And when i put a rifle with a scope attached in the slot, the scope is not visable.
You can see exactly this in my TrolleyKart and Veedub mod.
This is not just limited to rifles. Any item that has a attachment itself.
IE radio/glow stick on bags.