Bigger? That's a great size for a gun. You just need to up the contrast a bunch. A valuable lesson that usually takes plenty of time to figure out and that makes artists feel stupid when someone points out that they haven't figured it out yet...
If you look at the second photo you took, some parts of the marker seem positively black. Others, when the light hits in the right spot, look positively white. That's what I mean about contrast. Your whole gun is in the grey part of the spectrum as it is now. Spice it up a bit. Make shades pitch black (or at least really dark) and highlights shiny white, and you'll see that depth isn't hard to manage at all if you just dare use stronger shades.
