Can a person legally mount a fake gun on an amateur-built aircraft? Is there an FAA rule that states that no amateur-built aircraft are allowed to have a weapon mounted to it? (If so, I'd certainly like to see this rule and its conditions) Say I want to build an airplane that a friend and I made blueprints for a while ago, and it was about 60% fantasy. But the plane doesn't look the same without the guns. The guns really add a different feel to the plane, and to have no guns makes it feel as though it's missing something. But clearly I'd never use it. I just wanted to know if it is even legal to put a gun on an amateur-built aircraft,  which I'd assume it isn't. So to avoid any trouble, I think it'd be best not to put any functioning guns on my plane. So, this begs the question, Is it legal to put a fake gun on an airplane, or to put a functioning ball turret that has fake guns?
Empirically, yes it's legal. I've seen several reproduction WW1 aircraft myself at airshows with fake guns, missiles or bombs attached. And I was once at an EAA chapter meeting where a member demonstrated his fake gun 'firing' using compressed air. There are also factory-built aircraft with fake weapons added later, by the way.
As to regulations on this, I've never seen one. Typically US laws and regulations state what you can't do, not what you can do. For example, there is a law that prohibits attaching dangerous weapons to drones.