I'd suggest you reconsider the device of having the gun be plastic. When I lived out West, I owned a Glock handgun, and I assure you that although much of the frame is plastic, there still need to be plenty of metal components. So the spell would presumably transfigure them.
You might rescue the device if you were to say that the spell was intended to transfigure an entirely-metal object, and he hadn't counted on the gun's grips and frame being of plastic.
Also, the only plastic-framed gun on the market at the time of the Harry Potter stories was the Glock, which is made in Austria, not in America.