Sorry but i was thinking and my code it's not enough good.
I mean that it will remove weapon and even if you'll have nothing in hands then if you will change weapon and then change back to primary slot , it won't appear because it will be empty, or not.
I don't know , test it.
I know that there is an strip stock (fm, ham , engine... i dont know exactly) which strips the gun but it stays red weapon in primary slot and you can select it but nothing appears.It's what you need.
If someone knows how to do that then i can add it to my code.