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Lloyd Kaufman (yes, the guy from Troma) has some good insight into the MPAA in his book Make Your Own Damn Movie if I remember correctly. I believe he recounts how they (MPAA) wouldn't give a Troma movie a R rating because it had squibs in it. Lloyd then points out that a movie like Die Hard can have tons of squibs in it and still get a R rating.
Then again Lloyd is a little bit crazy so who knows if his word means anything.