I'm gonna go ahead and say this is most likely a made up quote.
In the first link provided (to Post-Modern Magik), after that quote it says
(see Muggles Do It Better)
In the literature examples of this page, we have the following:
Regarding weapons, there are extremely long debates, some on this very wiki, about how far this goes. It's generally argued[note] that a Muggle with a shotgun would usually beat a wizard...
Clicking the [note] expands the text and shows:
It's generally argued [note - at least by the fans; Rowling is surprisingly silent on the debate] that a Muggle with a shotgun would usually beat a wizard...
Unfortunately, the link goes to another TVTropes page and not anything meaningful. However, if the extra [note] is to be believed, JK hasn't actually said anything about whether a firearm would be useful against a Wizard.
I think the fact that we have so far been unable to find anything said by JK even remotely similar to "rational man with a shotgun" speaks for itself.