The problem here is that a lightmap isn't true shading, but its a modification of the level's texture to give the appearance of lights, or lack thereof. What you could do is have a single light, and move it to the closest imaginary lightsource, so it affects the gun. If a particular part of a room is supposed to be dark, you just dont place a light there, and the gun should go dark.
You might also want to turn ambient light down, as this can affect shading aswell.
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