Whats with the odd triangulations on the mesh? Specifically the tanks body. It almost looks like mesh errors that occur on a bad import or something...
Also whats the polycount? It looks like you have a lot of polys on the barrel and its mounting, as well as other areas like the hatch and such. Its difficult to critique a model without seeing wireframes. Assuming this is 3dsmax, which it appears to be due to the angle short cut in the top right, hit F4 on your keyboard so that we can see the wires over the mesh material.
Its not a bad start though. There are currently some inaccuracies with scaling/proportions, but your most likely aware of them since your pretty early into modeling it
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~Apple