other 3D programs but maybe they're similar? I dunno...
SETTING UP
First things first, you want to be in a map with one of those re-colourable white rooms. Set them to green or some colour that isn't on your gun so you will be able to crop the gun out easily. MAKE SURE YOU DON'T HAVE BLOOM OR ANTI-ALIASING ON, they'll mess up the edges of your model and make cropping it out harder. You'll want to have the gun in either neutral lighting or with the light slightly coming from the left to help make shading easier. Use the easy rotate mode on the physics gun to aim it easier (Shift+E) and position yourself in the right place. Once you are happy with the angle of the gun swap to the camera and hit print-screen. DON'T USE THE INGAME SCREENSHOT because it'll compress the image and you will lose quality.
STEP 1: Scaling the gun
So you got your screenshot. Crop out the gun and remove the solid colour background, then open up an empty 320x200 (428x200 for widescreen) canvas and scale the gun to fit nicely in it. You'll probably want to make a cross in the middle of the canvas to aim the weapon. Set the canvas colour to a contrasting colour so you can see the gun clearly against it.
STEP 2: Un-Anti-Aliasing but not kinda
Now you're going to want to make solid colour backgrounds around the blurry anti-aliased edges of the gun. Try to have it blend with the edges but not be the exact same colour. I usually have the edges be slightly darker than the rest of the gun. Once that is done flatten the layers (or if you're lazy like me copy paste the whole thing into paint then back into your editor) and magic wand out the edges with tolerance set to 0. You should have something like this.
STEP 3: Edges
Here's where you'll make the gun go from being a gross model rip to something kinda nice looking. First, go around and trim pixels so you get a nice edge around the gun.
After this, zoom in a bit closer and look for little artifacts from when you magic wand'd out the solid colour. You're going to want to take these pixels and either dodge them brighter or use the pencil tool to make the edges look nice.
BEFORE:
AFTER:
STEP 4: Shading
After you've gone around and made the edges look nice, take the whole sprite and make it grayscale.
You probably want to keep a coloured version of the gun somewhere for when you colour it. With the gun being black and white, use the dodge and burn tool to shade the gun. You generally want the light source to come from the left of the gun so it looks good with vanilla guns. I also like to highlight the edges and make fake ambient-occlusion. I'm not an authority on shading, so it'd probably be a good idea to look up innernet tutorials on it.
STEP 5: Colouring
Once you've got a nicely shaded gun, it's time to colour it in. There's probably better ways to do it but what I do is put solid colour patches over the parts of the gun I want to colour like this:
then I make it a colouring layer. You can also use this to apply camo to your gun, but you'll want to distort it slightly to make it look right. After that you're done. Now you can animate it or stuff.
THE FINISHED GUN:
