It really depends on how you're approaching it in some ways. When painting your lines are you using a hard brush with pen pressure on or off? I often like to have my pen pressure on (in other dynamics and also have pen pressure on for shape dynamics) so I can sketch light, like pencils, and then I go in and block colors and darken things. When I have the forms down I go in and erase the edges and keep working things out to get the lines smooth... IF I'm going for the clean rendered look. For quick concepts what you're doing more than rocks though.
Okay, random passerby here, but I'm telling you the exact same thing at the above happened to me for a short portion of my sophmore year in highschool, at which point I stopped drawing guns for a little while (the whole realization troubled me quite a bit, since then I've diverged into different drawing subjects aswell)