Shingles replacing shingles over gypsum board

I have read the messages about the two layers of Gypsum Board and figuring out how to install shingles over top. Can anyone tell me where to get the nails that dDubya talked about. I have a job that is 1200 sq. but 90 sq. of it has this same issue, two layers of Gypsum with I&W over top. I can’t find the nails anywhere. Does anyone know another application where we can still install the shingles on the roof and not have to pull the Gypsum board up? Can we go over it with a 7/16" OSB and screw through to the Metal Deck without changing the Fire-Rating?