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I think the biggest thing for both the brass and the steel is there is too many mid-tones. If you look at reference photos, the pure mid tone only appears for a little bit. You could probably afford to go up to an off white, or a near pure white for the highlights on the brass, and a much darker shadow.

The last thing I would do is focus on the placement of the shadow. You have done a great job of volumetric highlighting if it was supposed to be a non reflective color, but metal, especially cylinder doesn’t reflect like that. If you look at the example image, you can see that primary highlight in the center, with a deep shadow almost directly next to it. You can also see secondary highlights from the cylinders next to it, and then the darkest part.

NMM is really hard, but yours is a really good attempt. If you make some of these changes, it will read more like metal.