Your guns could fill the entire 1 -1 uv space and be on their own map if you wanted, making it bigger on a single map although not wrong would make it hard to create a texture that is the same scale as the other pieces on your uv map. You have to balance the time it would take to create a separate map with the detail seen and is it needed or worth it.
To create a metal look is is really all about the spec highlights, you mention that you didn't want it to be too shiny so you need to control that with spec maps.
With metal the defuse color tends be darker since metal tends to absorb more color so first i would darken your defuse map.
Try switching your shader type from phong to blinn in the attribute editor and then set the eccentricity to 0.2 and the roll off to 0.8 , this will give you very bright sharp highlights and a metal look but to control where you want the highlights you need a map for the specular color ( Black = no highlights and white = full highlights and grey the amounts inbetween) .