This is the method that I use:
1)Model the gun: I just model every model poly by poly.
2)Unwrap the gun: I use seems, even for structures.
3)Make the textures for the gun: diffuse map, normal map(optional), specular map(optional) and I use GIMP with some photos, a image of the UVs and some references.
4)Texture the gun: setup the materials(normally i use only one material, except for structures), setup the textures(diffuse map, normal map, specular map), ect.
Triangulate modifier? if you know what are you doing, you don’t need modifiers (except for mirror and armature).
When you start to model something, you need to know the max number of polys, if you pass the max number, you don’t need to use modifiers, there’re tools that can help you, like edge loop, or just delete some faces.
I apply all this to every model, I don’t care if the model is a gun, it can be a barrel, a car, a person, etc.