They look like vector traces of reference photos. That can be done in Illustrator, a little in Photoshop, and with the now-defunct Macromedia Freehand. You trace lines over the relevant bits of the gun then render out the details/shading/textures by hand. Do not get me wrong, this process is not automatic, and NOT by any means easy. The artist is dripping with talent.
Photoshop can "cartoonize" photos with the poster edges tool. I just grabbed a gun photo off Google images, and ran it through the filter, here's a before and after:
You can fiddle with the settings and go in and do a few other things by hand to clean it up and sharpen everything, but its not going to look much better than it does now.
The images still appear two dimensional for a variety of reasons. The biggest is comic-book style lines on the images. The shading methods he used are simplified and stylized, and that contributes as well. Some of the textures used, the wood stock on the top drawing is the best example, were not wrapped to the contours of the object, but laid on flat with shading on top. From the obvious skill in execution behind these drawings, I'd say the remaining two-dimensionality was the artist's choice, not a lack in talent.