hi clint,
let me be the first, and probably your worst nightmarefor a beginner your work is not half bad, biggest problem that stands out is your rivet detail, it needs reducing, are you using a seperate layer for the rivets, if so then make them more opaque, now your shadows, please keep them all consistant ie, all from the same angle, my personal taste is Not to curve the shadow but to keep it staight on a curved surface, but hey i am on my own here
technically my way is correct, but artistically a gentle curve looks nice, so make your own choice.
to understand the shadow in the rear gun cupola i suggest you study as many photos as you can and learn how the light and shade play in that area.
now the important stuff, i will be checking the drawing for accuracy so i hope it is right
by the way, i forgive you for being a Illustrator nerdlong live PSP7