Excellent, glad to see you're still working on it and improving!
Apologies about the size confusion. I found a dodgy jpg somewhere. It was blurry, as jpgs are, but it must also have been resized by about 10%. God knows why.
Anyway, I think it looks a lot better already, and I really prefer the outlined version. Helps to make it more distinct and clean I feel.
I still feel that it could be pushed and improved further. To that end I've tried my hand at my first ever edit of someone elses work. Hope you dont mind.

Made a fair few minor changes to the overall shape and shading. Have a close look and see how yours and mine differ, then take what you like and ditch what you dont.
The only thing I want to draw particular attention to is that I have 'grown' the wheels by a pixel vertically and horizontally. I dont know if you were planning to animate, but at the smaller size I think it would have been tricky make them spin and still retain the detail you were aiming for. And also if you look to your reference pictures, the front wheel extends as far forward as the gun does. Couldn't resist making a quick animation for you.
