Guns and other mechanical devices don't exist alone. A gun must interface with a bullet, a part of an aircraft must interface with other parts. So CAD AI must be able to understand the geometric context of the parts it is making.
That being said, I think AI will soon be capable of making mechanical devices. There has been some improvement in physical reasoning benchmarks like PHYRE[1]. Understanding physical reasoning and how multiple objects move with respect to each other is important in the synthesis of new mechanical devices.
SketchIt[0] demonstrated that by making reduced 2DoF description of how pairs of objects in a device may move with respect to each other, it's possible to synthesize a new device which performs the same function.
Solving PHYRE problems requires reasoning with larger degrees of freedom. The first example on the homepage has something like 5 objects which each have 3 positional DoF (translation and rotation). Even reasoning with 3DoF is quite difficult for approaches like those used in SketchIT.
Given that approaches like slotformer[2] already do somewhat well at solving these huge DoF problems, I don't think we're very far from AI being able to design complicated mechanical devices.
[0]https://dspace.mit.edu/bitstream/handle/1721.1/6773/AITR-157...