Well that seems like a real oversight on the part of Ultimate Survival! If the second camera (weapon camera) is on the default layer then this is likely part of the problem. Try making your own game layer called 'weapon', put your weapon objects on that layer, then set the weapon/fp camera to only render that layer with the clear flags set to 'depth only'. Then make sure this camera is set to forward rendering path, and that the depth number is 1 higher than the world camera.
Now in Suimono make sure this weapon layer is removed from the transparency layers list on the module object, as well as the reflections layer list on the surface object.