I'm not an expert on blender but how good are your, and I mean basic photoshop skills. it's amazing how many people still don't understand the best feature to the latest version of photoshop
If you designed that gun in blender or your really good at blender which I take it you are in asking for advice in blender instructions then I really think you either know photoshop front to back or you'll learn it in minutes
I paint all my diffusive textures in photoshop working directly with the 3D model itself and not 2D image. All you need is a UV map your satisfied with whether its complicated or not
Save a copy of your model in OBJ format and knowing photoshop just pick a texture format, if you've already used the trial and your trial has run out from an older version then this new version should work
Open the Wavefront Object in Photoshop and load the UV Map, open the UV map in another tab and frankly just start painting on the UV map, using layers if you want, when you click back over to your 3D object the changes are applied instantly and you'll see what you painted and how it looks exactly like it would in game
you can paint on the 3D model itself or tab back over to the unwrapped version and paint on that, either way the entire outcome and easiness is insanely better than any other alternative and you want have that problem
might take you about 5 ot 7 minutes to get the general idea of where the 3D stuff I mentined above here are in photoshop but I wouldn't know what to do without the new version
It doesn't matter about the 3D model in the end, after your done just save the texture and do whatever with the Wavefront Obj
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Advanced Licensed Computer Programmer and Custom Programming Language Interfacing. Graphics / Video Designer.