Armored Vehicle: Using Values and Perspective

For this task we were taught a few basics regarding form and values when lighting objects, such as "Half-way to black", the "1,2,3" on shading a cube and how to cast shadows from both local and natural light. Our task to carry out was to design a tank/armoured vehicle, as these would appear blocky enough to practice values on as well as putting the objects into perspective. First I made a moodboard to gather some research:

http://www.examiner.com/article/wildcat-robot-bizarre-looking-50-mph-robot-eerily-gallops-and-roams-freely (16/11/15)
http://www.rcmania.com/halo-rc-scorpion/ (16/11/15)
http://meckanicalmind.deviantart.com/art/Scout-Tank-243801482 (16/11/15)
http://eat3d.com/forum/work-progress/sci-fi-tank (16/11/15)
http://www.turbosquid.com/3d-model/science-fiction-tank/ (16/11/15)
http://www.beastsofwar.com/sci-fi-wargaming/dropzone-commander-introduces-heavy-hitting-land-forces/ (16/11/15)
http://peterprime.deviantart.com/art/Centurion-Tank-315786755 (16/11/15)
http://talros.deviantart.com/art/Mars-tank-SP-335136527 (16/11/15)
http://www.eurobricks.com/forum/index.php?showtopic=103494 (16/11/15)
http://www.beastsofwar.com/maxmini/maxmini-start-designing-scifi-gothic-tank/ (16/11/15)
http://www.valkyria.wikia.com (16/11/15)
https://www.pinterest.com/pin/398568635749873021/ (16/11/15)
http://digital-art-gallery.com/picture/big/239 (16/11/15)

I then went on to practice the lighting technique roughly, and learned that the sun position should be placed quite high. Then I designed an armoured vehicle based on the robot in the top left, however when I thought about how to put it into perspective I found it difficult, so I went on to design the tank and the hover lorry:

Out of these 2 I picked out the tank as the design would be easier to draw using perspective and show off different values, I then scanned this in and aligned the horizon and vanishing points in Photoshop:

Using Photoshop and Paint Tool SAI I went over the image digitally, and added in the shading by using black on another layer, and halfing the opacity, making it halfway to black. I also put in the shadows as well, however I did improvise for the gun barrel's shadow.

Despite the shadows looking a bit wonky, I feel that the values give the tank a strong sense of form, however I feel that the metalic parts could've been shaded better. In terms of perspective I feel this looks spot on.