Most of the weapon huds are done by taking a picture of the respective weapon, then converting the RGB colors to indexed colors using an appropriated color palette consisting in only 4-6 single colors per hud. I then change it back to RGB mode and fix some pixel noise by hand and paint some part by myself. Depending on the model, I sometimes also add a slight cartoon filter to underline a bit more some details. I am doing all this with The Gimp image editing software. Then I usually import the image in Inkscape and vectorize it, add a black outline and use a drop shadow to finalize it.