Uh, nifs already have a seperate texture. Look at the texture paths in the BSShaderLightingProperties.
sorry man... i guess i need to be completely re-schooled in how to make a custom weapon look, i use to do this back on Counter Strike source, but i have no clue on how to do it in Fallout: New vegas.
also i'm using gimp to edit the skin. (its editing the weapons look i am after, like changing the slide to look gold/silver whatever and then repackaging it and making a unique name so it doesn't change all the guns of the same type. )