Shiny metal is hard to draw, especially with two different metals together. So are inanimate objects and perspective. I usually shy away from drawing anything with strict straight lines, not to mention strict and accurate curves.
I searched for a random photo in my archive again today and this diver’s helmet came up. A nice thing about picking among my old photographs as references for drawings, is that I find that even crappy photos I can usually turn into a nice drawing. This photo was a disaster of over exposure, but it had it’s use after all. I think I was well out of my comfort zone here, but it was still great fun to draw. Now it’s a drawing that I like a lot.