The D600 has a magnesium top and back, but the front is plastic underneath the lens mounting ring. You can see a photo of it in the D600 review on photographylife.com. The author says to use common sense with heavy lenses and, "Will plastic bend if I let the D600 support a heavy lens? It might a little, but it will typically go back to its original shape. Whereas if metal bends, you know it stays there."