Gathering Reference - Gathering as much visual reference is often over looked by too many modelers or texture artists. The more you have the more small details you can find. I honestly can't stress how important this is. Even super close up images of a buttstock will show you just how smooth or rough a surface which will make your texture look that more realistic.
Reference Study - Take the time to look at your images you've collected. See which parts (in this case) are plastic and which parts are metal and make a list so your ready for the next step. Reference study is needed because as i said in the first step you may just find small details that will set your texture apart from others.
Material Properties - Some plastics are very reflective and some aren't. Some metals are covered with a tan coating and some aren't. All these factors need to be address early so you know how to create them once you start your texture. For instance, a plastic that is very dull and reflects very little light would look horrible if you made it very glossy looking and highly reflective.
-plastic - divide the list of plastic parts you made into different categories
-smooth
-rough
-dull
-glossy
-metal - divide the list of metal parts you made into different categories as well... for example
-brushed
-dull
-glossy or reflective
-worn/destroyed - only if you plan on having a destroyed look
-etc etc etc
Base Layer Creation - Create your base layer for your plastics and your metals... Sometimes a base layer you made for one previous vehicle or weapon will not work with this new weapon. That's the reason to study your reference material carefully. If you don't your texture will be off and may not get approved.
Highlights/Shadows - Add highlights and shadows to you texture to help add depth and a sense of realistic lighting to the texture. For instance, in a FPS game your 99% of the time going to have a light source above your head. SOOO that means the edges on top of an object are going to be highlighted and the bottom edges might be shaded. Trial and error for what looks the best is all i can give you
Sometimes you just don't know.Details - Adding dirt, scratches, nicks and weathering to a texture can sometimes make it look visually pleasing. You may not be going for this effect so it might not be needed.
Approval - This is normally the most nerve racking part. What your doing here is having people critique you on your skills. The hours you spent on something can be thrown out almost immediately. Sometimes you really have to swallow your pride and just say &%^$ it i'll do it over, but this time it's gonna be waaay better.
Don't rush your texture. It's better to take longer on a texture and have it kick *** than to do a quickie and have it look like a two year old put it together in windows paint.
I really hope this helps someone out there who already knows photoshop but doesn't know a good way to approach the pipeline of creating textures.