>I think< it is because the refined definition of what the actual 'gun' is. The polymer frame is not 'the gun.' >I think< that the barrel and slide are not defined as 'the gun.' Only the trigger mechanism and firing mechanism (pin, hammer, etc..) are considered 'the gun.' Which is why you can take a Sig P320 mechanism and put it into a different frame with a different barrel and slide (thereby changing the caliber) yet still have the same 'gun' i.e. modularity. Again, this is my very rudimentary understanding. Perhaps someone with more in-depth knowledge will chime in.