You can't hide/show nodes of a weapon image either.
It's done through static shapes I believe.
The way I was suggested to do it, was to use animations on the weapon. Since technically animations stay at the last frame of their little motion when completed, until the animation changes. (Or so I've seen); which means we can use stateSequence to change them.
Another way was to mount multiple images to the player at the same time, although this would require a lot of modeler cordination for each model, and offset expertise; and I don't know if you can actually mount more than one image to the player at any given time anyways.
There are a few others, I'd be interested in knowing how this individual did it though.
It appears people really like the idea of Ironsights, even though I told them I really don't think it's possible to do. I see myself working with something I don't like in the near future...